Release Calendar Tools 0.1.0

.gitignore

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+node_modules/
+dist/
+release/*.zip
+release/*.zip.sha256
+coverage/
+playwright-report/
+test-results/
+*.tsbuildinfo
+.DS_Store

.npmrc

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+@add-ideas:registry=https://git.add-ideas.de/api/packages/lotobo/npm/

CHANGELOG.md

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+# Changelog
+
+## 0.1.0 - 2026-09-01
+
+- Added the initial local-first Calendar Tools workbench.
+- Added iCalendar event/timezone inspection, bounded recurrence, conflict diagnostics, merge/deduplication, canonical repair, and DST gap/overlap analysis.
+- Added Toolbox integration, bounded input handling, offline shell and deterministic release packaging.

CONTRIBUTING.md

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+# Contributing
+
+Run `npm ci`, `npm run check`, and `npm run test:browser` before proposing a change. Keep processing local, preserve explicit limits, and include fixtures for every parser boundary.

LICENSE

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+  Later license versions may give you additional or different
+permissions.  However, no additional obligations are imposed on any
+author or copyright holder as a result of your choosing to follow a
+later version.
+
+  15. Disclaimer of Warranty.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
+APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT
+HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY
+OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO,
+THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM
+IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF
+ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
+
+  16. Limitation of Liability.
+
+  IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING
+WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS
+THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY
+GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE
+USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF
+DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD
+PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS),
+EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF
+SUCH DAMAGES.
+
+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.

LICENSES/README.md

@@ -0,0 +1,3 @@
+# Licences
+
+The application is GPL-3.0-or-later. Generated release artifacts include an exact runtime dependency licence inventory in `npm-runtime-licenses.txt`.

README.md

@@ -0,0 +1,34 @@
+# Calendar Tools
+
+Inspect, repair, and reconcile calendars locally in the browser.
+
+Calendar Tools is a standalone local-first application in the [add·ideas Toolbox](https://git.add-ideas.de/lotobo/toolbox-portal). Inputs are processed in the browser and are not uploaded.
+
+## Version 0.1 scope
+
+- RFC 5545 parsing and inspection for events, recurrence, attendees, calendar metadata, and embedded VTIMEZONE definitions
+- Bounded RRULE/RDATE/EXDATE occurrence preview and overlap/duplicate diagnostics
+- Merge and deduplication by UID plus RECURRENCE-ID, preferring SEQUENCE and then DTSTAMP
+- Conservative canonical repair: BOM/NUL removal, CRLF serialization, VERSION and PRODID normalization
+- Browser-IANA timezone offset/transition reports and explicit DST gap/overlap classification
+- Relocatable offline PWA, local-only processing, and deterministic release packaging
+
+Input is limited to 4 MiB, 100,000 physical lines, 10,000 components, and 5,000 events. Recurrence is limited to 500 occurrences per series, a 3,650-day horizon, and 5,000 displayed occurrences. See [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) and [docs/PRIVACY-SECURITY.md](docs/PRIVACY-SECURITY.md).
+
+## Development
+
+Requires Node.js 22 and npm 11.
+
+```sh
+npm ci
+npm run check
+npm run test:browser
+```
+
+## Release
+
+`npm run release:artifact` creates a deterministic `release/calendar-tools-0.1.0.zip` and checksum sidecar.
+
+## Licence
+
+GPL-3.0-or-later. Runtime dependencies retain their own licences; see [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md).

SECURITY.md

@@ -0,0 +1,3 @@
+# Security
+
+Please report vulnerabilities privately to the repository owner. Do not attach sensitive source files to public issues. Imported inputs are untrusted; parsers must remain bounded and must not execute active content or fetch external resources.

SOURCE.md

@@ -0,0 +1,7 @@
+# Corresponding source
+
+The corresponding source for Calendar Tools 0.1.0 is available at:
+
+https://git.add-ideas.de/lotobo/calendar-tools/src/tag/v0.1.0
+
+Build with Node.js 22, npm 11, `npm ci`, and `npm run release:artifact`.

THIRD_PARTY_NOTICES.md

@@ -0,0 +1,14 @@
+# Third-party notices
+
+Calendar Tools 0.1.0 directly depends on these runtime packages:
+
+| Package                          | Pinned version | Declared licence |
+| -------------------------------- | -------------: | ---------------- |
+| `@add-ideas/toolbox-contract`    |          0.2.3 | Apache-2.0       |
+| `@add-ideas/toolbox-helpers`     |          0.1.0 | GPL-3.0-or-later |
+| `@add-ideas/toolbox-shell-react` |          0.2.3 | Apache-2.0       |
+| `ical.js`                        |          2.2.1 | MPL-2.0          |
+| `react`                          |         19.2.8 | MIT              |
+| `react-dom`                      |         19.2.8 | MIT              |
+
+This table covers direct production dependencies, not transitive packages or development tooling. During a release build, the exact non-development dependency tree and discovered licence texts are generated from the lockfile into `LICENSES/npm-runtime-licenses.txt` and included in the release ZIP. Copyright and licence terms remain with their respective authors.

docs/ACCESSIBILITY.md

@@ -0,0 +1,3 @@
+# Accessibility
+
+The workbench uses semantic landmarks, headings and labelled native controls, supports keyboard operation, visible focus, reduced-width layouts, and system/light/dark themes through the Toolbox shell. Results do not rely on colour alone.

docs/ARCHITECTURE.md

@@ -0,0 +1,7 @@
+# Architecture
+
+Calendar Tools is a static React/Vite application wrapped in the shared Toolbox shell. `calendar/model.ts` places strict preflight bounds around the pinned `ical.js` parser, projects events and timezone definitions into inert inspection records, expands recurrence under explicit limits, and implements deterministic merge and repair operations.
+
+UID plus RECURRENCE-ID is the merge identity. Higher SEQUENCE wins; equal sequences use lexically later DTSTAMP. VTIMEZONE definitions are retained once per TZID. Conflict diagnostics operate on the bounded occurrence preview, not on an unbounded calendar.
+
+Embedded VTIMEZONE definitions are interpreted by `ical.js`. A TZID without an embedded definition is resolved through the browser's IANA data when available and is diagnosed as such. Floating times remain floating for display; conflict comparison assigns a UTC-shaped inspection coordinate and must not be treated as a scheduling conversion. Version 0.1 is synchronous, has no persistence or server API, and uses relative assets for nested portal paths.

docs/PRIVACY-SECURITY.md

@@ -0,0 +1,7 @@
+# Privacy and security
+
+Calendar source, parsed events, merge inputs and diagnostic output stay in page memory. There is no telemetry, analytics, account, persistence or runtime network path. Imported summaries, descriptions, locations, UIDs and organizer values are rendered as text, never executable markup.
+
+Files and pasted sources are rejected above 4 MiB; the preflight also caps physical lines, components and events. Recurrence has both per-series and time-horizon bounds and the conflict list is capped. These limits reduce accidental resource exhaustion but do not validate meeting claims or attendee identities.
+
+Repair intentionally normalizes only transport structure and required calendar metadata; it does not invent DTSTART, UID, timezone law or recurrence. Merge can discard an older component with the same identity, so its selection rule and count remain visible. Browser timezone data varies by runtime version; DST results are diagnostics, not a legal source of civil-time truth.

eslint.config.mjs

@@ -0,0 +1,37 @@
+import js from "@eslint/js";
+import globals from "globals";
+import reactHooks from "eslint-plugin-react-hooks";
+import reactRefresh from "eslint-plugin-react-refresh";
+import tseslint from "typescript-eslint";
+
+export default tseslint.config(
+  {
+    ignores: [
+      "dist",
+      "release",
+      "coverage",
+      "test-results",
+      "playwright-report",
+    ],
+  },
+  {
+    extends: [js.configs.recommended, ...tseslint.configs.recommended],
+    files: ["**/*.{ts,tsx}"],
+    languageOptions: {
+      ecmaVersion: 2023,
+      globals: { ...globals.browser, ...globals.worker },
+    },
+    plugins: { "react-hooks": reactHooks, "react-refresh": reactRefresh },
+    rules: {
+      ...reactHooks.configs.recommended.rules,
+      "react-refresh/only-export-components": [
+        "warn",
+        { allowConstantExport: true },
+      ],
+    },
+  },
+  {
+    files: ["scripts/**/*.mjs", "playwright.config.ts"],
+    languageOptions: { globals: globals.node },
+  },
+);

index.html

@@ -0,0 +1,19 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <meta
+      name="description"
+      content="Inspect, repair, and reconcile calendars locally in the browser."
+    />
+    <link rel="icon" type="image/svg+xml" href="./favicon.svg" />
+    <link rel="manifest" href="./manifest.webmanifest" />
+    <meta name="theme-color" content="#29255f" />
+    <title>Calendar Tools</title>
+  </head>
+  <body>
+    <div id="root"></div>
+    <script type="module" src="./src/main.tsx"></script>
+  </body>
+</html>

package.json

@@ -0,0 +1,72 @@
+{
+  "name": "calendar-tools",
+  "version": "0.1.0",
+  "description": "Inspect, repair, and reconcile calendars locally in the browser.",
+  "license": "GPL-3.0-or-later",
+  "author": "Albrecht Degering",
+  "repository": {
+    "type": "git",
+    "url": "git+https://git.add-ideas.de/lotobo/calendar-tools.git"
+  },
+  "homepage": "https://git.add-ideas.de/lotobo/calendar-tools",
+  "bugs": {
+    "url": "https://git.add-ideas.de/lotobo/calendar-tools/issues"
+  },
+  "private": true,
+  "type": "module",
+  "engines": {
+    "node": ">=22"
+  },
+  "scripts": {
+    "predev": "npm run manifest:generate",
+    "dev": "vite",
+    "prebuild": "npm run release:prepare && npm run manifest:generate",
+    "build": "tsc -b && vite build",
+    "preview": "vite preview",
+    "typecheck": "tsc -b --pretty false",
+    "lint": "eslint . --max-warnings=0",
+    "format": "prettier --write .",
+    "format:check": "prettier --check .",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:browser": "playwright test",
+    "manifest:generate": "node scripts/generate-toolbox-manifest.mjs",
+    "manifest:check": "node scripts/generate-toolbox-manifest.mjs --check",
+    "release:prepare": "node scripts/prepare-release-files.mjs",
+    "toolbox:check": "toolbox-check dist",
+    "package:release": "node scripts/package-release.mjs",
+    "check": "npm run manifest:check && npm run typecheck && npm run lint && npm run format:check && npm run test && npm run build && npm run toolbox:check",
+    "release:artifact": "npm run check && npm run test:browser && npm run package:release -- --force"
+  },
+  "dependencies": {
+    "@add-ideas/toolbox-contract": "0.2.3",
+    "@add-ideas/toolbox-helpers": "0.1.0",
+    "@add-ideas/toolbox-shell-react": "0.2.3",
+    "ical.js": "2.2.1",
+    "react": "19.2.8",
+    "react-dom": "19.2.8"
+  },
+  "devDependencies": {
+    "@add-ideas/toolbox-testkit": "0.2.3",
+    "@eslint/js": "10.0.1",
+    "@playwright/test": "1.62.1",
+    "@testing-library/jest-dom": "6.9.1",
+    "@testing-library/react": "16.3.2",
+    "@testing-library/user-event": "14.6.1",
+    "@types/node": "25.9.5",
+    "@types/react": "19.2.17",
+    "@types/react-dom": "19.2.5",
+    "@vitejs/plugin-react": "6.0.3",
+    "eslint": "10.7.0",
+    "eslint-plugin-react-hooks": "7.1.1",
+    "eslint-plugin-react-refresh": "0.5.2",
+    "globals": "17.7.0",
+    "jsdom": "29.1.1",
+    "prettier": "3.9.5",
+    "typescript": "6.0.3",
+    "typescript-eslint": "8.64.0",
+    "vite": "8.2.2",
+    "vitest": "4.1.11"
+  },
+  "packageManager": "npm@11.17.0"
+}

playwright.config.ts

@@ -0,0 +1,21 @@
+import { defineConfig, devices } from "@playwright/test";
+
+export default defineConfig({
+  testDir: "./tests/browser",
+  fullyParallel: false,
+  workers: 1,
+  timeout: 120_000,
+  expect: { timeout: 10_000 },
+  reporter: [["list"]],
+  use: { baseURL: "http://127.0.0.1:4173", trace: "retain-on-failure" },
+  webServer: {
+    command: "npm run build && node scripts/serve-test.mjs",
+    url: "http://127.0.0.1:4173",
+    reuseExistingServer: !process.env.CI,
+    timeout: 180_000,
+  },
+  projects: [
+    { name: "chromium", use: { ...devices["Desktop Chrome"] } },
+    { name: "firefox", use: { ...devices["Desktop Firefox"] } },
+  ],
+});

public/CHANGELOG.md

@@ -0,0 +1,7 @@
+# Changelog
+
+## 0.1.0 - 2026-09-01
+
+- Added the initial local-first Calendar Tools workbench.
+- Added iCalendar event/timezone inspection, bounded recurrence, conflict diagnostics, merge/deduplication, canonical repair, and DST gap/overlap analysis.
+- Added Toolbox integration, bounded input handling, offline shell and deterministic release packaging.

public/CONTRIBUTING.md

@@ -0,0 +1,3 @@
+# Contributing
+
+Run `npm ci`, `npm run check`, and `npm run test:browser` before proposing a change. Keep processing local, preserve explicit limits, and include fixtures for every parser boundary.

public/LICENSE

@@ -0,0 +1,674 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
+
+ Copyright (C) 2007 Free Software Foundation, Inc. <https://fsf.org/>
+ Everyone is permitted to copy and distribute verbatim copies
+ of this license document, but changing it is not allowed.
+
+                            Preamble
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+
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public/LICENSES/README.md

@@ -0,0 +1,3 @@
+# Licences
+
+The application is GPL-3.0-or-later. Generated release artifacts include an exact runtime dependency licence inventory in `npm-runtime-licenses.txt`.

public/README.md

@@ -0,0 +1,34 @@
+# Calendar Tools
+
+Inspect, repair, and reconcile calendars locally in the browser.
+
+Calendar Tools is a standalone local-first application in the [add·ideas Toolbox](https://git.add-ideas.de/lotobo/toolbox-portal). Inputs are processed in the browser and are not uploaded.
+
+## Version 0.1 scope
+
+- RFC 5545 parsing and inspection for events, recurrence, attendees, calendar metadata, and embedded VTIMEZONE definitions
+- Bounded RRULE/RDATE/EXDATE occurrence preview and overlap/duplicate diagnostics
+- Merge and deduplication by UID plus RECURRENCE-ID, preferring SEQUENCE and then DTSTAMP
+- Conservative canonical repair: BOM/NUL removal, CRLF serialization, VERSION and PRODID normalization
+- Browser-IANA timezone offset/transition reports and explicit DST gap/overlap classification
+- Relocatable offline PWA, local-only processing, and deterministic release packaging
+
+Input is limited to 4 MiB, 100,000 physical lines, 10,000 components, and 5,000 events. Recurrence is limited to 500 occurrences per series, a 3,650-day horizon, and 5,000 displayed occurrences. See [docs/ARCHITECTURE.md](docs/ARCHITECTURE.md) and [docs/PRIVACY-SECURITY.md](docs/PRIVACY-SECURITY.md).
+
+## Development
+
+Requires Node.js 22 and npm 11.
+
+```sh
+npm ci
+npm run check
+npm run test:browser
+```
+
+## Release
+
+`npm run release:artifact` creates a deterministic `release/calendar-tools-0.1.0.zip` and checksum sidecar.
+
+## Licence
+
+GPL-3.0-or-later. Runtime dependencies retain their own licences; see [THIRD_PARTY_NOTICES.md](THIRD_PARTY_NOTICES.md).

public/SECURITY.md

@@ -0,0 +1,3 @@
+# Security
+
+Please report vulnerabilities privately to the repository owner. Do not attach sensitive source files to public issues. Imported inputs are untrusted; parsers must remain bounded and must not execute active content or fetch external resources.

public/SOURCE.md

@@ -0,0 +1,7 @@
+# Corresponding source
+
+The corresponding source for Calendar Tools 0.1.0 is available at:
+
+https://git.add-ideas.de/lotobo/calendar-tools/src/tag/v0.1.0
+
+Build with Node.js 22, npm 11, `npm ci`, and `npm run release:artifact`.

public/THIRD_PARTY_NOTICES.md

@@ -0,0 +1,14 @@
+# Third-party notices
+
+Calendar Tools 0.1.0 directly depends on these runtime packages:
+
+| Package                          | Pinned version | Declared licence |
+| -------------------------------- | -------------: | ---------------- |
+| `@add-ideas/toolbox-contract`    |          0.2.3 | Apache-2.0       |
+| `@add-ideas/toolbox-helpers`     |          0.1.0 | GPL-3.0-or-later |
+| `@add-ideas/toolbox-shell-react` |          0.2.3 | Apache-2.0       |
+| `ical.js`                        |          2.2.1 | MPL-2.0          |
+| `react`                          |         19.2.8 | MIT              |
+| `react-dom`                      |         19.2.8 | MIT              |
+
+This table covers direct production dependencies, not transitive packages or development tooling. During a release build, the exact non-development dependency tree and discovered licence texts are generated from the lockfile into `LICENSES/npm-runtime-licenses.txt` and included in the release ZIP. Copyright and licence terms remain with their respective authors.

public/docs/ACCESSIBILITY.md

@@ -0,0 +1,3 @@
+# Accessibility
+
+The workbench uses semantic landmarks, headings and labelled native controls, supports keyboard operation, visible focus, reduced-width layouts, and system/light/dark themes through the Toolbox shell. Results do not rely on colour alone.

public/docs/ARCHITECTURE.md

@@ -0,0 +1,7 @@
+# Architecture
+
+Calendar Tools is a static React/Vite application wrapped in the shared Toolbox shell. `calendar/model.ts` places strict preflight bounds around the pinned `ical.js` parser, projects events and timezone definitions into inert inspection records, expands recurrence under explicit limits, and implements deterministic merge and repair operations.
+
+UID plus RECURRENCE-ID is the merge identity. Higher SEQUENCE wins; equal sequences use lexically later DTSTAMP. VTIMEZONE definitions are retained once per TZID. Conflict diagnostics operate on the bounded occurrence preview, not on an unbounded calendar.
+
+Embedded VTIMEZONE definitions are interpreted by `ical.js`. A TZID without an embedded definition is resolved through the browser's IANA data when available and is diagnosed as such. Floating times remain floating for display; conflict comparison assigns a UTC-shaped inspection coordinate and must not be treated as a scheduling conversion. Version 0.1 is synchronous, has no persistence or server API, and uses relative assets for nested portal paths.

public/docs/PRIVACY-SECURITY.md

@@ -0,0 +1,7 @@
+# Privacy and security
+
+Calendar source, parsed events, merge inputs and diagnostic output stay in page memory. There is no telemetry, analytics, account, persistence or runtime network path. Imported summaries, descriptions, locations, UIDs and organizer values are rendered as text, never executable markup.
+
+Files and pasted sources are rejected above 4 MiB; the preflight also caps physical lines, components and events. Recurrence has both per-series and time-horizon bounds and the conflict list is capped. These limits reduce accidental resource exhaustion but do not validate meeting claims or attendee identities.
+
+Repair intentionally normalizes only transport structure and required calendar metadata; it does not invent DTSTART, UID, timezone law or recurrence. Merge can discard an older component with the same identity, so its selection rule and count remain visible. Browser timezone data varies by runtime version; DST results are diagnostics, not a legal source of civil-time truth.

public/favicon.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" role="img" aria-label="Calendar Tools"><rect width="54" height="50" x="5" y="9" rx="7" fill="#5b4ec4"/><path fill="#fff" d="M5 22h54v7H5z" opacity=".35"/><path stroke="#fff" stroke-linecap="round" stroke-width="5" d="M18 5v11M46 5v11"/><path fill="#fff" d="M16 34h8v8h-8zm12 0h8v8h-8zm12 0h8v8h-8zM16 46h8v8h-8zm12 0h8v8h-8z"/></svg>

public/manifest.webmanifest

@@ -0,0 +1,18 @@
+{
+  "name": "Calendar Tools",
+  "short_name": "Calendar",
+  "description": "Inspect, repair, and reconcile calendars locally in the browser.",
+  "start_url": "./",
+  "scope": "./",
+  "display": "standalone",
+  "background_color": "#f6f7fb",
+  "theme_color": "#29255f",
+  "icons": [
+    {
+      "src": "./favicon.svg",
+      "sizes": "any",
+      "type": "image/svg+xml",
+      "purpose": "any maskable"
+    }
+  ]
+}

public/sw.js

@@ -0,0 +1,54 @@
+const CACHE_PREFIX = "calendar-tools-shell-";
+const CACHE_NAME = CACHE_PREFIX + "0.1.0";
+const CORE = ["./", "./manifest.webmanifest", "./favicon.svg"];
+self.addEventListener("install", (event) => {
+  event.waitUntil(
+    caches.open(CACHE_NAME).then(async (cache) => {
+      await cache.addAll(CORE);
+      const html = await (await fetch("./")).text();
+      const assets = [...html.matchAll(/(?:src|href)="(\.\/assets\/[^"]+)"/g)]
+        .map((match) => match[1])
+        .filter(Boolean);
+      await cache.addAll(assets);
+    }),
+  );
+  self.skipWaiting();
+});
+self.addEventListener("activate", (event) => {
+  event.waitUntil(
+    caches
+      .keys()
+      .then((keys) =>
+        Promise.all(
+          keys
+            .filter((key) => key.startsWith(CACHE_PREFIX) && key !== CACHE_NAME)
+            .map((key) => caches.delete(key)),
+        ),
+      )
+      .then(() => self.clients.claim()),
+  );
+});
+self.addEventListener("fetch", (event) => {
+  if (event.request.method !== "GET") return;
+  const url = new URL(event.request.url);
+  if (
+    url.origin !== self.location.origin ||
+    !url.pathname.startsWith(new URL(self.registration.scope).pathname)
+  )
+    return;
+  event.respondWith(
+    caches.match(event.request).then(
+      (cached) =>
+        cached ??
+        fetch(event.request).then((response) => {
+          if (response.ok) {
+            const copy = response.clone();
+            void caches
+              .open(CACHE_NAME)
+              .then((cache) => cache.put(event.request, copy));
+          }
+          return response;
+        }),
+    ),
+  );
+});

public/toolbox-app.json

@@ -0,0 +1,41 @@
+{
+  "$schema": "https://git.add-ideas.de/lotobo/toolbox-sdk/raw/branch/main/schemas/toolbox-app.v1.schema.json",
+  "schemaVersion": 1,
+  "id": "de.add-ideas.calendar-tools",
+  "name": "Calendar Tools",
+  "version": "0.1.0",
+  "description": "Inspect, repair, and reconcile calendars locally in the browser.",
+  "entry": "./",
+  "icon": "./favicon.svg",
+  "categories": ["productivity", "data", "time"],
+  "tags": ["calendar", "ics", "icalendar", "rrule", "timezone", "dst", "merge"],
+  "integration": {
+    "contextVersion": 1,
+    "launchModes": ["navigate", "new-tab"],
+    "embedding": "unsupported"
+  },
+  "requirements": {
+    "secureContext": false,
+    "workers": false,
+    "indexedDb": false,
+    "crossOriginIsolated": false,
+    "topLevelContext": false
+  },
+  "privacy": {
+    "processing": "local",
+    "fileUploads": true,
+    "telemetry": false,
+    "label": "Inputs stay in this browser; nothing is uploaded."
+  },
+  "source": {
+    "repository": "https://git.add-ideas.de/lotobo/calendar-tools",
+    "license": "GPL-3.0-or-later"
+  },
+  "actions": [
+    {
+      "id": "source",
+      "label": "Source",
+      "url": "https://git.add-ideas.de/lotobo/calendar-tools"
+    }
+  ]
+}

scripts/generate-toolbox-manifest.mjs

@@ -0,0 +1,33 @@
+import { readFile, writeFile } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+import { format } from "prettier";
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const sourcePath = path.join(root, "src/toolbox/manifest.source.json");
+const outputPath = path.join(root, "public/toolbox-app.json");
+const source = JSON.parse(await readFile(sourcePath, "utf8"));
+const pkg = JSON.parse(await readFile(path.join(root, "package.json"), "utf8"));
+const versionSource = await readFile(path.join(root, "src/version.ts"), "utf8");
+const appVersion = /^export const APP_VERSION = "([^"]+)";$/mu.exec(
+  versionSource,
+)?.[1];
+const repository = "https://git.add-ideas.de/lotobo/" + pkg.name;
+if (source.version !== pkg.version || appVersion !== pkg.version)
+  throw new Error("Version identity drift");
+if (
+  source.id !== "de.add-ideas." + pkg.name ||
+  source.source?.repository !== repository ||
+  source.source?.license !== "GPL-3.0-or-later"
+)
+  throw new Error("Manifest source identity is invalid");
+const serialized = await format(JSON.stringify(source), {
+  filepath: outputPath,
+});
+if (process.argv.includes("--check")) {
+  if ((await readFile(outputPath, "utf8").catch(() => "")) !== serialized)
+    throw new Error("public/toolbox-app.json is stale");
+  console.log("Toolbox manifest is synchronized");
+} else {
+  await writeFile(outputPath, serialized);
+  console.log("Generated public/toolbox-app.json");
+}

scripts/package-release.mjs

@@ -0,0 +1,165 @@
+#!/usr/bin/env node
+import { createHash } from "node:crypto";
+import { execFile } from "node:child_process";
+import {
+  access,
+  chmod,
+  copyFile,
+  cp,
+  lstat,
+  mkdir,
+  mkdtemp,
+  readFile,
+  readdir,
+  rename,
+  rm,
+  utimes,
+  writeFile,
+} from "node:fs/promises";
+import os from "node:os";
+import path from "node:path";
+import { promisify } from "node:util";
+import { fileURLToPath } from "node:url";
+const execute = promisify(execFile);
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const pkg = JSON.parse(await readFile(path.join(root, "package.json"), "utf8"));
+const argument = (name, fallback) => {
+  const index = process.argv.indexOf(name);
+  return index >= 0 ? process.argv[index + 1] : fallback;
+};
+const output = path.resolve(
+  root,
+  argument("--output", "release/" + pkg.name + "-" + pkg.version + ".zip"),
+);
+const checksumOutput = output + ".sha256";
+const force = process.argv.includes("--force");
+if (
+  path.extname(output).toLowerCase() !== ".zip" ||
+  output === root ||
+  output === path.parse(output).root
+)
+  throw new Error("Unsafe release target");
+const exists = (file) =>
+  access(file).then(
+    () => true,
+    () => false,
+  );
+if (!force && ((await exists(output)) || (await exists(checksumOutput))))
+  throw new Error("Release output exists");
+const input = path.join(root, "dist");
+for (const name of [
+  "index.html",
+  "manifest.webmanifest",
+  "sw.js",
+  "toolbox-app.json",
+  "favicon.svg",
+  "README.md",
+  "CHANGELOG.md",
+  "CONTRIBUTING.md",
+  "LICENSE",
+  "SECURITY.md",
+  "SOURCE.md",
+  "THIRD_PARTY_NOTICES.md",
+  "LICENSES/README.md",
+  "LICENSES/npm-runtime-licenses.txt",
+  "docs/ACCESSIBILITY.md",
+  "docs/ARCHITECTURE.md",
+  "docs/PRIVACY-SECURITY.md",
+]) {
+  const details = await lstat(path.join(input, name)).catch(() => null);
+  if (!details?.isFile() || details.isSymbolicLink())
+    throw new Error("Missing release file: " + name);
+}
+const manifest = JSON.parse(
+  await readFile(path.join(input, "toolbox-app.json"), "utf8"),
+);
+const repository = "https://git.add-ideas.de/lotobo/" + pkg.name;
+if (
+  manifest.id !== "de.add-ideas." + pkg.name ||
+  manifest.version !== pkg.version ||
+  manifest.entry !== "./" ||
+  manifest.icon !== "./favicon.svg" ||
+  manifest.source?.repository !== repository
+)
+  throw new Error("Packaged manifest identity is invalid");
+if (
+  /\b(?:src|href)=["']\//iu.test(
+    await readFile(path.join(input, "index.html"), "utf8"),
+  )
+)
+  throw new Error("Root-absolute asset reference");
+async function collect(directory, prefix = "") {
+  const files = [];
+  for (const entry of (await readdir(directory, { withFileTypes: true })).sort(
+    (a, b) => a.name.localeCompare(b.name),
+  )) {
+    const absolute = path.join(directory, entry.name);
+    const relative = prefix ? prefix + "/" + entry.name : entry.name;
+    if (entry.isSymbolicLink())
+      throw new Error("Symlink in release: " + relative);
+    if (entry.isDirectory()) files.push(...(await collect(absolute, relative)));
+    else if (entry.isFile()) files.push({ absolute, relative });
+    else throw new Error("Unsupported release entry: " + relative);
+  }
+  return files;
+}
+const sourceFiles = await collect(input);
+for (const file of sourceFiles)
+  if (
+    file.relative.endsWith(".map") ||
+    /(?:^|\/)(?:\.env(?:\.|$)|id_rsa|id_ed25519|.*\.pem$|.*\.key$)/iu.test(
+      file.relative,
+    ) ||
+    file.relative.split("/").includes("..")
+  )
+    throw new Error("Forbidden release entry: " + file.relative);
+await mkdir(path.dirname(output), { recursive: true });
+const stagingRoot = await mkdtemp(
+  path.join(os.tmpdir(), pkg.name + "-release-"),
+);
+const publicationRoot = await mkdtemp(
+  path.join(path.dirname(output), "." + pkg.name + "-publish-"),
+);
+const stagedTree = path.join(stagingRoot, "tree");
+const stagedArchive = path.join(stagingRoot, path.basename(output));
+try {
+  await cp(input, stagedTree, { recursive: true });
+  const timestamp = new Date("1980-01-01T00:00:00.000Z");
+  for (const file of await collect(stagedTree)) {
+    await chmod(file.absolute, 0o644);
+    await utimes(file.absolute, timestamp, timestamp);
+  }
+  await execute(
+    "zip",
+    [
+      "-X",
+      "-q",
+      "-9",
+      stagedArchive,
+      ...sourceFiles.map((file) => file.relative),
+    ],
+    {
+      cwd: stagedTree,
+      env: { ...process.env, TZ: "UTC" },
+      maxBuffer: 1024 * 1024,
+    },
+  );
+  const archive = await readFile(stagedArchive);
+  const digest = createHash("sha256").update(archive).digest("hex");
+  const stagedChecksum = stagedArchive + ".sha256";
+  await writeFile(stagedChecksum, digest + "  " + path.basename(output) + "\n");
+  const publicationArchive = path.join(publicationRoot, path.basename(output));
+  const publicationChecksum = publicationArchive + ".sha256";
+  await copyFile(stagedArchive, publicationArchive);
+  await copyFile(stagedChecksum, publicationChecksum);
+  if (force) {
+    await rm(output, { force: true });
+    await rm(checksumOutput, { force: true });
+  }
+  await rename(publicationArchive, output);
+  await rename(publicationChecksum, checksumOutput);
+  console.log("Created " + path.relative(root, output) + "\nSHA-256 " + digest);
+} finally {
+  await rm(stagingRoot, { recursive: true, force: true });
+  await rm(publicationRoot, { recursive: true, force: true });
+}

scripts/prepare-release-files.mjs

@@ -0,0 +1,70 @@
+import { cp, mkdir, readFile, readdir, rm, writeFile } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+const root = path.resolve(path.dirname(fileURLToPath(import.meta.url)), "..");
+const destination = path.join(root, "public");
+for (const name of [
+  "LICENSE",
+  "README.md",
+  "CHANGELOG.md",
+  "CONTRIBUTING.md",
+  "SECURITY.md",
+  "SOURCE.md",
+  "THIRD_PARTY_NOTICES.md",
+]) {
+  await readFile(path.join(root, name));
+  await cp(path.join(root, name), path.join(destination, name));
+}
+for (const directory of ["LICENSES", "docs"]) {
+  const output = path.join(destination, directory);
+  await rm(output, { recursive: true, force: true });
+  await cp(path.join(root, directory), output, { recursive: true });
+}
+const lock = JSON.parse(
+  await readFile(path.join(root, "package-lock.json"), "utf8"),
+);
+const sections = [];
+for (const [location, locked] of Object.entries(lock.packages ?? {}).sort(
+  ([a], [b]) => a.localeCompare(b),
+)) {
+  if (!location.includes("node_modules/") || locked.dev === true) continue;
+  const packageDirectory = path.join(root, location);
+  const details = JSON.parse(
+    await readFile(path.join(packageDirectory, "package.json"), "utf8"),
+  );
+  const candidates = (await readdir(packageDirectory))
+    .filter((name) => /^(?:licen[cs]e|copying|notice)(?:\.|$)/iu.test(name))
+    .sort();
+  const texts = [];
+  for (const candidate of candidates) {
+    try {
+      texts.push(
+        "--- " +
+          candidate +
+          " ---\n" +
+          (await readFile(path.join(packageDirectory, candidate), "utf8")),
+      );
+    } catch {
+      /* directory */
+    }
+  }
+  sections.push(
+    "=".repeat(78) +
+      "\n" +
+      details.name +
+      "@" +
+      details.version +
+      "\nDeclared licence: " +
+      (details.license ?? locked.license ?? "See upstream") +
+      "\n" +
+      "=".repeat(78) +
+      "\n" +
+      (texts.join("\n\n") || "See upstream package metadata."),
+  );
+}
+await mkdir(path.join(destination, "LICENSES"), { recursive: true });
+await writeFile(
+  path.join(destination, "LICENSES/npm-runtime-licenses.txt"),
+  sections.join("\n\n").trimEnd() + "\n",
+);
+console.log("Prepared release documentation");

scripts/serve-test.mjs

@@ -0,0 +1,66 @@
+import { createServer } from "node:http";
+import { readFile, stat } from "node:fs/promises";
+import path from "node:path";
+import { fileURLToPath } from "node:url";
+const root = path.resolve(
+  path.dirname(fileURLToPath(import.meta.url)),
+  "..",
+  "dist",
+);
+const prefix = "/deep/nested/calendar/";
+const types = new Map([
+  [".css", "text/css; charset=utf-8"],
+  [".html", "text/html; charset=utf-8"],
+  [".js", "text/javascript; charset=utf-8"],
+  [".json", "application/json; charset=utf-8"],
+  [".webmanifest", "application/manifest+json; charset=utf-8"],
+  [".svg", "image/svg+xml"],
+  [".md", "text/markdown; charset=utf-8"],
+  [".txt", "text/plain; charset=utf-8"],
+  [".wasm", "application/wasm"],
+  [".png", "image/png"],
+  [".jpg", "image/jpeg"],
+  [".webp", "image/webp"],
+]);
+const headers = {
+  "Content-Security-Policy":
+    "default-src 'self'; base-uri 'self'; object-src 'none'; frame-ancestors 'none'; form-action 'self'; script-src 'self' 'wasm-unsafe-eval'; style-src 'self' 'unsafe-inline'; img-src 'self' data: blob:; media-src 'self' blob:; connect-src 'self'; worker-src 'self' blob:; manifest-src 'self'",
+  "Cross-Origin-Opener-Policy": "same-origin",
+  "Cross-Origin-Resource-Policy": "same-origin",
+  "Permissions-Policy":
+    "camera=(), microphone=(), geolocation=(), usb=(), payment=()",
+  "Referrer-Policy": "no-referrer",
+  "X-Content-Type-Options": "nosniff",
+};
+const server = createServer(async (request, response) => {
+  try {
+    const url = new URL(request.url ?? "/", "http://127.0.0.1");
+    const relative = decodeURIComponent(url.pathname).startsWith(prefix)
+      ? decodeURIComponent(url.pathname).slice(prefix.length)
+      : decodeURIComponent(url.pathname).replace(/^\/+/, "");
+    const normalized = path.posix.normalize(relative || "index.html");
+    if (
+      normalized === ".." ||
+      normalized.startsWith("../") ||
+      path.isAbsolute(normalized)
+    ) {
+      response.writeHead(400).end("Bad request");
+      return;
+    }
+    let file = path.join(root, normalized);
+    if ((await stat(file).catch(() => null))?.isDirectory())
+      file = path.join(file, "index.html");
+    const content = await readFile(file);
+    response.writeHead(200, {
+      "Content-Type":
+        types.get(path.extname(file)) ?? "application/octet-stream",
+      "Cache-Control": "no-cache",
+      ...headers,
+    });
+    response.end(content);
+  } catch {
+    response.writeHead(404, { "Content-Type": "text/plain; charset=utf-8" });
+    response.end("Not found");
+  }
+});
+server.listen(4173, "127.0.0.1", () => console.log("Test server ready"));

src/App.tsx

@@ -0,0 +1,35 @@
+import { lazy, Suspense, useState } from "react";
+import { AppShell } from "@add-ideas/toolbox-shell-react";
+import "@add-ideas/toolbox-shell-react/styles.css";
+import "./styles.css";
+import { ErrorBoundary } from "./components/ErrorBoundary";
+import { HelpDialog } from "./components/HelpDialog";
+import { manifest } from "./toolbox/manifest";
+
+const Workbench = lazy(async () => ({
+  default: (await import("./components/Workbench")).Workbench,
+}));
+
+export function App() {
+  const [helpOpen, setHelpOpen] = useState(false);
+  return (
+    <ErrorBoundary>
+      <AppShell
+        app={manifest}
+        manifestUrl="./toolbox-app.json"
+        helpAction={{ onClick: () => setHelpOpen(true) }}
+      >
+        <Suspense
+          fallback={
+            <p className="loading" role="status">
+              Preparing Calendar Tools…
+            </p>
+          }
+        >
+          <Workbench />
+        </Suspense>
+      </AppShell>
+      <HelpDialog open={helpOpen} onClose={() => setHelpOpen(false)} />
+    </ErrorBoundary>
+  );
+}

src/calendar/model.ts

@@ -0,0 +1,630 @@
+import ICAL from "ical.js";
+
+export interface CalendarDiagnostic {
+  severity: "info" | "warning" | "error";
+  message: string;
+  uid?: string;
+}
+
+export interface CalendarEvent {
+  id: string;
+  uid: string;
+  recurrenceId: string;
+  summary: string;
+  description: string;
+  location: string;
+  status: string;
+  organizer: string;
+  attendees: number;
+  startText: string;
+  endText: string;
+  startMs: number | null;
+  endMs: number | null;
+  allDay: boolean;
+  timezone: string;
+  recurring: boolean;
+  recurrenceRules: string[];
+  sequence: number;
+  stamp: string;
+  component: InstanceType<typeof ICAL.Component>;
+}
+
+export interface TimezoneRecord {
+  tzid: string;
+  observances: number;
+  standard: number;
+  daylight: number;
+  lastModified: string;
+}
+
+export interface CalendarDocument {
+  component: InstanceType<typeof ICAL.Component>;
+  events: CalendarEvent[];
+  timezones: TimezoneRecord[];
+  diagnostics: CalendarDiagnostic[];
+  properties: { prodid: string; version: string; method: string; name: string };
+}
+
+export interface Occurrence {
+  uid: string;
+  summary: string;
+  startText: string;
+  endText: string;
+  startMs: number | null;
+  endMs: number | null;
+  timezone: string;
+  allDay: boolean;
+}
+
+export interface Conflict {
+  left: Occurrence;
+  right: Occurrence;
+  kind: "overlap" | "duplicate";
+}
+
+const MAX_SOURCE = 4 * 1024 * 1024;
+const MAX_LINES = 100_000;
+const MAX_COMPONENTS = 10_000;
+const MAX_EVENTS = 5_000;
+
+function textValue(
+  component: InstanceType<typeof ICAL.Component>,
+  name: string,
+): string {
+  const value = component.getFirstPropertyValue(name);
+  return typeof value === "string" ? value : (value?.toString() ?? "");
+}
+
+function formatLocalParts(epochMs: number, zone: string): string {
+  const formatter = new Intl.DateTimeFormat("en-CA", {
+    timeZone: zone,
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+    hour: "2-digit",
+    minute: "2-digit",
+    second: "2-digit",
+    hourCycle: "h23",
+  });
+  const parts = Object.fromEntries(
+    formatter
+      .formatToParts(new Date(epochMs))
+      .filter((part) => part.type !== "literal")
+      .map((part) => [part.type, part.value]),
+  );
+  return `${parts.year}-${parts.month}-${parts.day}T${parts.hour}:${parts.minute}:${parts.second}`;
+}
+
+function offsetAt(epochMs: number, zone: string): number {
+  const local = formatLocalParts(epochMs, zone);
+  const match = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2}):(\d{2})$/u.exec(
+    local,
+  );
+  if (!match) throw new Error(`Could not format time in ${zone}.`);
+  const interpreted = Date.UTC(
+    Number(match[1]),
+    Number(match[2]) - 1,
+    Number(match[3]),
+    Number(match[4]),
+    Number(match[5]),
+    Number(match[6]),
+  );
+  return Math.round((interpreted - epochMs) / 60_000);
+}
+
+export function localTimeCandidates(local: string, zone: string): number[] {
+  const match = /^(\d{4})-(\d{2})-(\d{2})T(\d{2}):(\d{2})(?::(\d{2}))?$/u.exec(
+    local,
+  );
+  if (!match) throw new Error("Local time must use YYYY-MM-DDTHH:mm[:ss].");
+  const base = Date.UTC(
+    Number(match[1]),
+    Number(match[2]) - 1,
+    Number(match[3]),
+    Number(match[4]),
+    Number(match[5]),
+    Number(match[6] ?? 0),
+  );
+  const offsets = new Set<number>();
+  for (let delta = -48; delta <= 48; delta += 6)
+    offsets.add(offsetAt(base + delta * 3_600_000, zone));
+  const canonical = `${match[1]}-${match[2]}-${match[3]}T${match[4]}:${match[5]}:${match[6] ?? "00"}`;
+  return [...offsets]
+    .map((offset) => base - offset * 60_000)
+    .filter((candidate) => formatLocalParts(candidate, zone) === canonical)
+    .sort((left, right) => left - right);
+}
+
+function calendarTimeText(time: InstanceType<typeof ICAL.Time>): string {
+  const year = String(time.year).padStart(4, "0");
+  const month = String(time.month).padStart(2, "0");
+  const day = String(time.day).padStart(2, "0");
+  if (time.isDate) return `${year}-${month}-${day}`;
+  return `${year}-${month}-${day}T${String(time.hour).padStart(2, "0")}:${String(time.minute).padStart(2, "0")}:${String(time.second).padStart(2, "0")}`;
+}
+
+function resolveTime(
+  time: InstanceType<typeof ICAL.Time>,
+  tzid: string,
+  hasDefinition: boolean,
+): number | null {
+  if (time.isDate) return Date.UTC(time.year, time.month - 1, time.day);
+  if (time.zone?.tzid === "UTC" || tzid.toUpperCase() === "UTC")
+    return time.toJSDate().getTime();
+  if (hasDefinition && time.zone?.tzid && time.zone.tzid !== "floating")
+    return time.toJSDate().getTime();
+  if (tzid) {
+    try {
+      return localTimeCandidates(calendarTimeText(time), tzid)[0] ?? null;
+    } catch {
+      return null;
+    }
+  }
+  return Date.UTC(
+    time.year,
+    time.month - 1,
+    time.day,
+    time.hour,
+    time.minute,
+    time.second,
+  );
+}
+
+function preflight(source: string) {
+  if (source.length > MAX_SOURCE)
+    throw new Error("Calendar input exceeds the 4 MiB limit.");
+  const lines = source
+    .replaceAll("\r\n", "\n")
+    .replaceAll("\r", "\n")
+    .split("\n");
+  if (lines.length > MAX_LINES)
+    throw new Error("Calendar input exceeds the 100,000-line limit.");
+  const components = lines.filter((line) => /^BEGIN:/iu.test(line)).length;
+  if (components > MAX_COMPONENTS)
+    throw new Error("Calendar input exceeds the 10,000-component limit.");
+}
+
+function eventFromComponent(
+  component: InstanceType<typeof ICAL.Component>,
+  index: number,
+  timezoneIds: Set<string>,
+  diagnostics: CalendarDiagnostic[],
+): CalendarEvent {
+  const event = new ICAL.Event(component);
+  const startProperty = component.getFirstProperty("dtstart");
+  if (!startProperty) throw new Error(`VEVENT ${index + 1} has no DTSTART.`);
+  const start = event.startDate;
+  const end = event.endDate;
+  const tzidParameter = startProperty.getParameter("tzid");
+  const timezone =
+    typeof tzidParameter === "string"
+      ? tzidParameter
+      : start.zone?.tzid === "UTC"
+        ? "UTC"
+        : "floating";
+  const hasDefinition = timezoneIds.has(timezone);
+  const startMs = resolveTime(
+    start,
+    timezone === "floating" ? "" : timezone,
+    hasDefinition,
+  );
+  const endMs = resolveTime(
+    end,
+    timezone === "floating" ? "" : timezone,
+    hasDefinition,
+  );
+  const uid = event.uid || `missing-uid-${index + 1}`;
+  if (!event.uid)
+    diagnostics.push({
+      severity: "warning",
+      uid,
+      message:
+        "VEVENT has no UID; a deterministic inspection-only identifier was assigned.",
+    });
+  if (
+    timezone &&
+    timezone !== "floating" &&
+    timezone !== "UTC" &&
+    !hasDefinition
+  ) {
+    try {
+      new Intl.DateTimeFormat("en", { timeZone: timezone }).format(0);
+      diagnostics.push({
+        severity: "info",
+        uid,
+        message: `${timezone} has no embedded VTIMEZONE; preview uses this browser's IANA timezone data.`,
+      });
+    } catch {
+      diagnostics.push({
+        severity: "warning",
+        uid,
+        message: `${timezone} has no embedded definition and is unknown to this browser; absolute-time diagnostics are unavailable.`,
+      });
+    }
+  }
+  if (startMs === null || endMs === null)
+    diagnostics.push({
+      severity: "warning",
+      uid,
+      message:
+        "The event falls in an invalid or unresolved local time; absolute conflict checks skip it.",
+    });
+  const recurrenceRules = component
+    .getAllProperties("rrule")
+    .map((property) => property.getFirstValue()?.toString() ?? "");
+  return {
+    id: `${uid}:${textValue(component, "recurrence-id") || "master"}:${index}`,
+    uid,
+    recurrenceId: textValue(component, "recurrence-id"),
+    summary: event.summary || "(untitled event)",
+    description: event.description || "",
+    location: event.location || "",
+    status: textValue(component, "status"),
+    organizer: event.organizer || "",
+    attendees: event.attendees.length,
+    startText: calendarTimeText(start),
+    endText: calendarTimeText(end),
+    startMs,
+    endMs,
+    allDay: start.isDate,
+    timezone,
+    recurring: event.isRecurring(),
+    recurrenceRules,
+    sequence: event.sequence || 0,
+    stamp: textValue(component, "dtstamp"),
+    component,
+  };
+}
+
+export function parseCalendar(source: string): CalendarDocument {
+  preflight(source);
+  let component: InstanceType<typeof ICAL.Component>;
+  try {
+    component = new ICAL.Component(ICAL.parse(source.replace(/^\uFEFF/u, "")));
+  } catch (reason) {
+    throw new Error(
+      `iCalendar parse failed: ${reason instanceof Error ? reason.message : "invalid syntax"}`,
+      { cause: reason },
+    );
+  }
+  if (component.name !== "vcalendar")
+    throw new Error("The root component must be VCALENDAR.");
+  const diagnostics: CalendarDiagnostic[] = [];
+  const zones = component.getAllSubcomponents("vtimezone");
+  const timezoneIds = new Set(
+    zones.map((zone) => textValue(zone, "tzid")).filter(Boolean),
+  );
+  const eventComponents = component.getAllSubcomponents("vevent");
+  if (eventComponents.length > MAX_EVENTS)
+    throw new Error("Calendar input exceeds the 5,000-event limit.");
+  const events: CalendarEvent[] = [];
+  for (const [index, eventComponent] of eventComponents.entries()) {
+    try {
+      events.push(
+        eventFromComponent(eventComponent, index, timezoneIds, diagnostics),
+      );
+    } catch (reason) {
+      diagnostics.push({
+        severity: "error",
+        message:
+          reason instanceof Error
+            ? reason.message
+            : `Could not inspect VEVENT ${index + 1}.`,
+      });
+    }
+  }
+  const version = textValue(component, "version");
+  const prodid = textValue(component, "prodid");
+  if (version !== "2.0")
+    diagnostics.push({
+      severity: "warning",
+      message: `VERSION is ${version || "missing"}; canonical repair writes 2.0.`,
+    });
+  if (!prodid)
+    diagnostics.push({
+      severity: "warning",
+      message:
+        "PRODID is missing; canonical repair supplies a local identifier.",
+    });
+  const timezones = zones.map((zone) => ({
+    tzid: textValue(zone, "tzid") || "(missing TZID)",
+    observances: zone
+      .getAllSubcomponents()
+      .filter((item) => item.name === "standard" || item.name === "daylight")
+      .length,
+    standard: zone.getAllSubcomponents("standard").length,
+    daylight: zone.getAllSubcomponents("daylight").length,
+    lastModified: textValue(zone, "last-modified"),
+  }));
+  return {
+    component,
+    events,
+    timezones,
+    diagnostics,
+    properties: {
+      prodid,
+      version,
+      method: textValue(component, "method"),
+      name: textValue(component, "x-wr-calname"),
+    },
+  };
+}
+
+function occurrenceFromTime(
+  event: CalendarEvent,
+  time: InstanceType<typeof ICAL.Time>,
+  durationMs: number,
+  timezoneIds: Set<string>,
+): Occurrence {
+  const startText = calendarTimeText(time);
+  const startMs = resolveTime(
+    time,
+    event.timezone === "floating" ? "" : event.timezone,
+    timezoneIds.has(event.timezone),
+  );
+  return {
+    uid: event.uid,
+    summary: event.summary,
+    startText,
+    endText:
+      startMs === null
+        ? "unresolved"
+        : new Date(startMs + durationMs).toISOString(),
+    startMs,
+    endMs: startMs === null ? null : startMs + durationMs,
+    timezone: event.timezone,
+    allDay: event.allDay,
+  };
+}
+
+export function previewOccurrences(
+  document: CalendarDocument,
+  limit = 100,
+  horizonDays = 730,
+): { occurrences: Occurrence[]; truncated: boolean } {
+  if (!Number.isInteger(limit) || limit < 1 || limit > 500)
+    throw new Error("Occurrence limit must be 1–500.");
+  if (!Number.isInteger(horizonDays) || horizonDays < 1 || horizonDays > 3_650)
+    throw new Error("Recurrence horizon must be 1–3,650 days.");
+  const timezoneIds = new Set(document.timezones.map((zone) => zone.tzid));
+  const occurrences: Occurrence[] = [];
+  let truncated = false;
+  for (const event of document.events.filter((entry) => !entry.recurrenceId)) {
+    const duration =
+      event.startMs !== null && event.endMs !== null
+        ? Math.max(0, event.endMs - event.startMs)
+        : 0;
+    if (!event.recurring) {
+      occurrences.push({
+        uid: event.uid,
+        summary: event.summary,
+        startText: event.startText,
+        endText: event.endText,
+        startMs: event.startMs,
+        endMs: event.endMs,
+        timezone: event.timezone,
+        allDay: event.allDay,
+      });
+      continue;
+    }
+    const iterator = new ICAL.Event(event.component).iterator();
+    const startBound = event.startMs ?? 0;
+    const horizon = startBound + horizonDays * 86_400_000;
+    for (let count = 0; count < limit; count += 1) {
+      const next = iterator.next();
+      if (!next) break;
+      const occurrence = occurrenceFromTime(event, next, duration, timezoneIds);
+      if (occurrence.startMs !== null && occurrence.startMs > horizon) break;
+      occurrences.push(occurrence);
+      if (count === limit - 1) truncated = true;
+    }
+  }
+  occurrences.sort(
+    (left, right) =>
+      (left.startMs ?? Number.MAX_SAFE_INTEGER) -
+        (right.startMs ?? Number.MAX_SAFE_INTEGER) ||
+      left.summary.localeCompare(right.summary),
+  );
+  if (occurrences.length > 5_000) {
+    occurrences.length = 5_000;
+    truncated = true;
+  }
+  return { occurrences, truncated };
+}
+
+function eventIdentity(
+  component: InstanceType<typeof ICAL.Component>,
+): string | null {
+  const uid = textValue(component, "uid");
+  return uid ? `${uid}|${textValue(component, "recurrence-id")}` : null;
+}
+
+function newerEvent(
+  left: InstanceType<typeof ICAL.Component>,
+  right: InstanceType<typeof ICAL.Component>,
+): InstanceType<typeof ICAL.Component> {
+  const leftSequence = Number(textValue(left, "sequence") || 0);
+  const rightSequence = Number(textValue(right, "sequence") || 0);
+  if (rightSequence !== leftSequence)
+    return rightSequence > leftSequence ? right : left;
+  return textValue(right, "dtstamp") > textValue(left, "dtstamp")
+    ? right
+    : left;
+}
+
+export function mergeCalendars(documents: readonly CalendarDocument[]): {
+  source: string;
+  diagnostics: CalendarDiagnostic[];
+  deduplicated: number;
+} {
+  if (documents.length < 1 || documents.length > 20)
+    throw new Error("Merge accepts 1–20 calendars.");
+  const root = new ICAL.Component("vcalendar");
+  root.addPropertyWithValue("version", "2.0");
+  root.addPropertyWithValue("prodid", "-//add·ideas//Calendar Tools 0.1.0//EN");
+  const zones = new Map<string, InstanceType<typeof ICAL.Component>>();
+  const events = new Map<string, InstanceType<typeof ICAL.Component>>();
+  let deduplicated = 0;
+  let missingUids = 0;
+  for (const [documentIndex, document] of documents.entries()) {
+    for (const zone of document.component.getAllSubcomponents("vtimezone")) {
+      const id = textValue(zone, "tzid");
+      if (id && !zones.has(id)) zones.set(id, zone);
+    }
+    for (const [eventIndex, event] of document.component
+      .getAllSubcomponents("vevent")
+      .entries()) {
+      const declaredIdentity = eventIdentity(event);
+      const identity =
+        declaredIdentity ?? `~missing-${documentIndex}-${eventIndex}`;
+      if (!declaredIdentity) missingUids += 1;
+      const previous = events.get(identity);
+      if (previous) {
+        events.set(identity, newerEvent(previous, event));
+        deduplicated += 1;
+      } else events.set(identity, event);
+    }
+  }
+  for (const zone of [...zones.values()].sort((a, b) =>
+    textValue(a, "tzid").localeCompare(textValue(b, "tzid")),
+  ))
+    root.addSubcomponent(new ICAL.Component(structuredClone(zone.toJSON())));
+  for (const [, event] of [...events.entries()].sort(([left], [right]) =>
+    left.localeCompare(right),
+  ))
+    root.addSubcomponent(new ICAL.Component(structuredClone(event.toJSON())));
+  return {
+    source:
+      root.toString().replaceAll("\n", "\r\n").replaceAll("\r\r\n", "\r\n") +
+      (root.toString().endsWith("\n") ? "" : "\r\n"),
+    diagnostics: [
+      {
+        severity: "info",
+        message: `Merged ${documents.length} calendars; retained ${events.size} event identities and ${zones.size} timezone definitions.`,
+      },
+      ...(missingUids
+        ? [
+            {
+              severity: "warning" as const,
+              message: `${missingUids} UID-less events were retained independently because they have no safe cross-calendar identity.`,
+            },
+          ]
+        : []),
+    ],
+    deduplicated,
+  };
+}
+
+export function findConflicts(
+  occurrences: readonly Occurrence[],
+  limit = 500,
+): Conflict[] {
+  const usable = occurrences
+    .filter((entry) => entry.startMs !== null && entry.endMs !== null)
+    .slice(0, 5_000)
+    .sort((a, b) => a.startMs! - b.startMs!);
+  const conflicts: Conflict[] = [];
+  for (let leftIndex = 0; leftIndex < usable.length; leftIndex += 1) {
+    const left = usable[leftIndex]!;
+    for (
+      let rightIndex = leftIndex + 1;
+      rightIndex < usable.length;
+      rightIndex += 1
+    ) {
+      const right = usable[rightIndex]!;
+      if (right.startMs! >= left.endMs!) break;
+      if (left.uid === right.uid && left.startMs === right.startMs)
+        conflicts.push({ left, right, kind: "duplicate" });
+      else if (left.uid !== right.uid && right.endMs! > left.startMs!)
+        conflicts.push({ left, right, kind: "overlap" });
+      if (conflicts.length >= limit) return conflicts;
+    }
+  }
+  return conflicts;
+}
+
+export function canonicalRepair(source: string): {
+  source: string;
+  repairs: string[];
+} {
+  preflight(source);
+  const repairs: string[] = [];
+  let normalized = source
+    .replace(/^\uFEFF/u, "")
+    .replaceAll("\0", "")
+    .replaceAll("\r\n", "\n")
+    .replaceAll("\r", "\n");
+  if (normalized !== source)
+    repairs.push("Removed BOM/NUL bytes and normalized physical line endings.");
+  const parsed = parseCalendar(normalized);
+  const clone = new ICAL.Component(structuredClone(parsed.component.toJSON()));
+  if (textValue(clone, "version") !== "2.0") {
+    clone.updatePropertyWithValue("version", "2.0");
+    repairs.push("Set VERSION:2.0.");
+  }
+  if (!textValue(clone, "prodid")) {
+    clone.addPropertyWithValue(
+      "prodid",
+      "-//add·ideas//Calendar Tools 0.1.0//EN",
+    );
+    repairs.push("Added PRODID.");
+  }
+  normalized = clone.toString();
+  normalized = normalized
+    .replaceAll("\r\n", "\n")
+    .replaceAll("\r", "\n")
+    .replaceAll("\n", "\r\n");
+  if (!normalized.endsWith("\r\n")) normalized += "\r\n";
+  if (!repairs.length)
+    repairs.push("Re-serialized the calendar into canonical folded CRLF form.");
+  return { source: normalized, repairs };
+}
+
+export interface ZoneTransition {
+  at: string;
+  fromMinutes: number;
+  toMinutes: number;
+}
+export function timezoneDiagnostics(
+  zone: string,
+  year: number,
+): {
+  transitions: ZoneTransition[];
+  januaryOffset: number;
+  julyOffset: number;
+} {
+  if (!Number.isInteger(year) || year < 1970 || year > 2100)
+    throw new Error("Timezone year must be 1970–2100.");
+  new Intl.DateTimeFormat("en", { timeZone: zone }).format(0);
+  const transitions: ZoneTransition[] = [];
+  let previousAt = Date.UTC(year, 0, 1);
+  let previous = offsetAt(previousAt, zone);
+  for (
+    let at = previousAt + 6 * 3_600_000;
+    at < Date.UTC(year + 1, 0, 2);
+    at += 6 * 3_600_000
+  ) {
+    const next = offsetAt(at, zone);
+    if (next !== previous) {
+      let low = previousAt;
+      let high = at;
+      while (high - low > 60_000) {
+        const middle = Math.floor((low + high) / 120_000) * 60_000;
+        if (offsetAt(middle, zone) === previous) low = middle;
+        else high = middle;
+      }
+      transitions.push({
+        at: new Date(high).toISOString(),
+        fromMinutes: previous,
+        toMinutes: next,
+      });
+      previous = next;
+    }
+    previousAt = at;
+  }
+  return {
+    transitions,
+    januaryOffset: offsetAt(Date.UTC(year, 0, 15, 12), zone),
+    julyOffset: offsetAt(Date.UTC(year, 6, 15, 12), zone),
+  };
+}

src/components/ErrorBoundary.tsx

@@ -0,0 +1,27 @@
+import { Component, type ErrorInfo, type ReactNode } from "react";
+
+export class ErrorBoundary extends Component<
+  { children: ReactNode },
+  { error?: Error }
+> {
+  state: { error?: Error } = {};
+  static getDerivedStateFromError(error: Error) {
+    return { error };
+  }
+  componentDidCatch(error: Error, info: ErrorInfo) {
+    console.error("Application failure", error, info);
+  }
+  render() {
+    if (this.state.error)
+      return (
+        <main className="fatal">
+          <h1>Calendar Tools could not continue</h1>
+          <p>{this.state.error.message}</p>
+          <button type="button" onClick={() => location.reload()}>
+            Reload
+          </button>
+        </main>
+      );
+    return this.props.children;
+  }
+}

src/components/HelpDialog.tsx

@@ -0,0 +1,41 @@
+import { useEffect, useRef } from "react";
+
+export function HelpDialog({
+  open,
+  onClose,
+}: {
+  open: boolean;
+  onClose: () => void;
+}) {
+  const dialog = useRef<HTMLDialogElement>(null);
+  useEffect(() => {
+    const node = dialog.current;
+    if (!node) return;
+    if (open && !node.open) node.showModal();
+    if (!open && node.open) node.close();
+  }, [open]);
+  return (
+    <dialog
+      ref={dialog}
+      className="help-dialog"
+      onClose={onClose}
+      onCancel={onClose}
+      aria-labelledby="help-title"
+    >
+      <div className="dialog-heading">
+        <div>
+          <p className="eyebrow">Local-first help</p>
+          <h2 id="help-title">About Calendar Tools</h2>
+        </div>
+        <button type="button" onClick={onClose} aria-label="Close help">
+          ×
+        </button>
+      </div>
+      <p>Inspect, repair, and reconcile calendars locally in the browser.</p>
+      <p>
+        All processing is performed in this browser. Imported data is treated as
+        untrusted and bounded before parsing.
+      </p>
+    </dialog>
+  );
+}

src/components/Workbench.tsx

@@ -0,0 +1,577 @@
+import { useMemo, useState } from "react";
+import { triggerBlobDownload } from "@add-ideas/toolbox-helpers";
+import {
+  canonicalRepair,
+  findConflicts,
+  localTimeCandidates,
+  mergeCalendars,
+  parseCalendar,
+  previewOccurrences,
+  timezoneDiagnostics,
+  type CalendarDocument,
+} from "../calendar/model";
+
+const sample = `BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//add ideas//Calendar Tools sample//EN\r\nX-WR-CALNAME:Project calendar\r\nBEGIN:VEVENT\r\nUID:weekly-review@example.test\r\nDTSTAMP:20260801T080000Z\r\nDTSTART;TZID=Europe/Berlin:20260901T100000\r\nDTEND;TZID=Europe/Berlin:20260901T110000\r\nRRULE:FREQ=WEEKLY;COUNT=6;BYDAY=TU\r\nSUMMARY:Weekly review\r\nLOCATION:Studio\r\nEND:VEVENT\r\nBEGIN:VEVENT\r\nUID:design-session@example.test\r\nDTSTAMP:20260801T080000Z\r\nDTSTART:20260901T103000Z\r\nDTEND:20260901T120000Z\r\nSUMMARY:Design session\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n`;
+
+const secondSample = `BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//add ideas//Second sample//EN\r\nBEGIN:VEVENT\r\nUID:weekly-review@example.test\r\nSEQUENCE:2\r\nDTSTAMP:20260802T080000Z\r\nDTSTART;TZID=Europe/Berlin:20260901T103000\r\nDTEND;TZID=Europe/Berlin:20260901T113000\r\nRRULE:FREQ=WEEKLY;COUNT=6;BYDAY=TU\r\nSUMMARY:Updated weekly review\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n`;
+
+type Tab = "inspect" | "recurrence" | "merge" | "timezone";
+
+function downloadIcs(value: string, name = "calendar.ics") {
+  triggerBlobDownload(
+    new Blob([value], { type: "text/calendar;charset=utf-8" }),
+    name,
+  );
+}
+
+function formatInstant(value: number | null, fallback: string) {
+  return value === null ? fallback : new Date(value).toISOString();
+}
+
+function offsetLabel(minutes: number) {
+  const sign = minutes < 0 ? "−" : "+";
+  const absolute = Math.abs(minutes);
+  return `UTC${sign}${String(Math.floor(absolute / 60)).padStart(2, "0")}:${String(absolute % 60).padStart(2, "0")}`;
+}
+
+export function Workbench() {
+  const initial = parseCalendar(sample);
+  const [tab, setTab] = useState<Tab>("inspect");
+  const [source, setSource] = useState(sample);
+  const [document, setDocument] = useState<CalendarDocument>(initial);
+  const [error, setError] = useState("");
+  const [repairs, setRepairs] = useState<string[]>([]);
+  const [limit, setLimit] = useState(100);
+  const [horizon, setHorizon] = useState(730);
+  const [occurrenceState, setOccurrenceState] = useState(() =>
+    previewOccurrences(initial, 100, 730),
+  );
+  const [mergeLeft, setMergeLeft] = useState(sample);
+  const [mergeRight, setMergeRight] = useState(secondSample);
+  const [mergeOutput, setMergeOutput] = useState("");
+  const [mergeInfo, setMergeInfo] = useState("");
+  const [zone, setZone] = useState("Europe/Berlin");
+  const [zoneYear, setZoneYear] = useState(2026);
+  const [localTime, setLocalTime] = useState("2026-10-25T02:30");
+  const [zoneResult, setZoneResult] = useState(() =>
+    timezoneDiagnostics("Europe/Berlin", 2026),
+  );
+  const [candidates, setCandidates] = useState(() =>
+    localTimeCandidates("2026-10-25T02:30", "Europe/Berlin"),
+  );
+  const conflicts = useMemo(
+    () => findConflicts(occurrenceState.occurrences),
+    [occurrenceState],
+  );
+
+  const accept = (value: string) => {
+    const next = parseCalendar(value);
+    setDocument(next);
+    setOccurrenceState(previewOccurrences(next, limit, horizon));
+    setRepairs([]);
+    setError("");
+  };
+  const parse = () => {
+    try {
+      accept(source);
+    } catch (reason) {
+      setError(
+        reason instanceof Error
+          ? reason.message
+          : "Could not inspect the calendar.",
+      );
+    }
+  };
+  const open = async (file: File | undefined) => {
+    if (!file) return;
+    if (file.size > 4 * 1024 * 1024) {
+      setError("Calendar file exceeds the 4 MiB limit.");
+      return;
+    }
+    const value = await file.text();
+    setSource(value);
+    try {
+      accept(value);
+    } catch (reason) {
+      setError(
+        reason instanceof Error
+          ? reason.message
+          : "Could not inspect the calendar file.",
+      );
+    }
+  };
+  const repair = () => {
+    try {
+      const repaired = canonicalRepair(source);
+      setSource(repaired.source);
+      setRepairs(repaired.repairs);
+      accept(repaired.source);
+      setRepairs(repaired.repairs);
+    } catch (reason) {
+      setError(
+        reason instanceof Error ? reason.message : "Calendar repair failed.",
+      );
+    }
+  };
+  const expand = () => {
+    try {
+      setOccurrenceState(previewOccurrences(document, limit, horizon));
+      setError("");
+    } catch (reason) {
+      setError(
+        reason instanceof Error ? reason.message : "Recurrence preview failed.",
+      );
+    }
+  };
+  const merge = () => {
+    try {
+      const result = mergeCalendars([
+        parseCalendar(mergeLeft),
+        parseCalendar(mergeRight),
+      ]);
+      setMergeOutput(result.source);
+      setMergeInfo(
+        `${result.deduplicated} duplicate event identit${result.deduplicated === 1 ? "y" : "ies"} resolved by SEQUENCE, then DTSTAMP.`,
+      );
+      setError("");
+    } catch (reason) {
+      setError(reason instanceof Error ? reason.message : "Merge failed.");
+    }
+  };
+  const inspectZone = () => {
+    try {
+      setZoneResult(timezoneDiagnostics(zone, zoneYear));
+      setCandidates(localTimeCandidates(localTime, zone));
+      setError("");
+    } catch (reason) {
+      setError(
+        reason instanceof Error
+          ? reason.message
+          : "Timezone inspection failed.",
+      );
+    }
+  };
+
+  return (
+    <main className="workbench">
+      <header className="hero">
+        <div>
+          <p className="eyebrow">Calendar evidence, not guesswork</p>
+          <h1>Calendar Tools</h1>
+          <p>
+            Inspect iCalendar structure, preview bounded recurrence, reconcile
+            files, and diagnose local times without uploading schedules.
+          </p>
+        </div>
+        <span className="privacy-pill">Browser-local</span>
+      </header>
+      {error && (
+        <p className="alert" role="alert">
+          {error}
+        </p>
+      )}
+      <nav className="workspace-tabs" aria-label="Calendar workspaces">
+        {(["inspect", "recurrence", "merge", "timezone"] as const).map(
+          (value) => (
+            <button
+              key={value}
+              type="button"
+              role="tab"
+              aria-selected={tab === value}
+              onClick={() => setTab(value)}
+            >
+              {value[0]!.toUpperCase() + value.slice(1)}
+            </button>
+          ),
+        )}
+      </nav>
+
+      {tab === "inspect" && (
+        <div className="split-layout">
+          <section className="panel workspace">
+            <div className="panel-heading">
+              <div>
+                <p className="eyebrow">RFC 5545 source</p>
+                <h2>Calendar input</h2>
+              </div>
+              <label className="button file-button">
+                Open .ics
+                <input
+                  type="file"
+                  accept=".ics,text/calendar"
+                  onChange={(event) => void open(event.target.files?.[0])}
+                />
+              </label>
+            </div>
+            <textarea
+              aria-label="iCalendar source"
+              value={source}
+              onChange={(event) => setSource(event.target.value)}
+              spellCheck={false}
+            />
+            <div className="actions">
+              <button type="button" className="primary" onClick={parse}>
+                Inspect calendar
+              </button>
+              <button type="button" onClick={repair}>
+                Repair and canonicalize
+              </button>
+              <button type="button" onClick={() => downloadIcs(source)}>
+                Download current .ics
+              </button>
+            </div>
+            <p className="muted">
+              Failed parses leave the last successful inspection visible. Repair
+              never guesses missing event times.
+            </p>
+            {repairs.length > 0 && (
+              <ul className="notice-list">
+                {repairs.map((item) => (
+                  <li key={item}>{item}</li>
+                ))}
+              </ul>
+            )}
+          </section>
+          <section className="panel workspace">
+            <div className="facts">
+              <div>
+                <span>Events</span>
+                <strong>{document.events.length}</strong>
+              </div>
+              <div>
+                <span>Recurring</span>
+                <strong>
+                  {document.events.filter((event) => event.recurring).length}
+                </strong>
+              </div>
+              <div>
+                <span>VTIMEZONE</span>
+                <strong>{document.timezones.length}</strong>
+              </div>
+              <div>
+                <span>Diagnostics</span>
+                <strong>{document.diagnostics.length}</strong>
+              </div>
+            </div>
+            <dl className="metadata">
+              <div>
+                <dt>Calendar name</dt>
+                <dd>{document.properties.name || "—"}</dd>
+              </div>
+              <div>
+                <dt>PRODID</dt>
+                <dd>{document.properties.prodid || "missing"}</dd>
+              </div>
+              <div>
+                <dt>VERSION / METHOD</dt>
+                <dd>
+                  {document.properties.version || "missing"} /{" "}
+                  {document.properties.method || "none"}
+                </dd>
+              </div>
+            </dl>
+            <div className="table-scroll">
+              <table>
+                <thead>
+                  <tr>
+                    <th>Event</th>
+                    <th>Start</th>
+                    <th>Zone</th>
+                    <th>Rule</th>
+                  </tr>
+                </thead>
+                <tbody>
+                  {document.events.map((event) => (
+                    <tr key={event.id}>
+                      <td>
+                        <strong>{event.summary}</strong>
+                        <span>{event.location || event.uid}</span>
+                      </td>
+                      <td>{formatInstant(event.startMs, event.startText)}</td>
+                      <td>{event.timezone}</td>
+                      <td>{event.recurrenceRules.join("; ") || "single"}</td>
+                    </tr>
+                  ))}
+                </tbody>
+              </table>
+            </div>
+            <details>
+              <summary>
+                Calendar and timezone diagnostics (
+                {document.diagnostics.length + document.timezones.length})
+              </summary>
+              <ul className="diagnostics">
+                {document.timezones.map((timezone) => (
+                  <li key={timezone.tzid}>
+                    {timezone.tzid}: {timezone.standard} STANDARD,{" "}
+                    {timezone.daylight} DAYLIGHT observances
+                  </li>
+                ))}
+                {document.diagnostics.map((diagnostic, index) => (
+                  <li
+                    key={`${diagnostic.uid ?? "calendar"}-${index}`}
+                    data-severity={diagnostic.severity}
+                  >
+                    {diagnostic.uid ? `${diagnostic.uid}: ` : ""}
+                    {diagnostic.message}
+                  </li>
+                ))}
+              </ul>
+            </details>
+          </section>
+        </div>
+      )}
+
+      {tab === "recurrence" && (
+        <section className="panel workspace">
+          <div className="panel-heading">
+            <div>
+              <p className="eyebrow">Explicitly bounded expansion</p>
+              <h2>Occurrence and conflict preview</h2>
+            </div>
+            <span>
+              {occurrenceState.occurrences.length} occurrences ·{" "}
+              {conflicts.length} conflicts
+            </span>
+          </div>
+          <div className="control-row">
+            <label className="field">
+              <span>Per-series limit</span>
+              <input
+                type="number"
+                min="1"
+                max="500"
+                value={limit}
+                onChange={(event) => setLimit(Number(event.target.value))}
+              />
+            </label>
+            <label className="field">
+              <span>Horizon (days)</span>
+              <input
+                type="number"
+                min="1"
+                max="3650"
+                value={horizon}
+                onChange={(event) => setHorizon(Number(event.target.value))}
+              />
+            </label>
+            <button type="button" className="primary" onClick={expand}>
+              Refresh preview
+            </button>
+          </div>
+          {occurrenceState.truncated && (
+            <p className="warning">
+              Preview reached a per-series or global bound; later occurrences
+              are intentionally not shown.
+            </p>
+          )}
+          <div className="timeline">
+            {occurrenceState.occurrences.map((occurrence, index) => (
+              <article
+                key={`${occurrence.uid}-${occurrence.startText}-${index}`}
+              >
+                <time>
+                  {formatInstant(occurrence.startMs, occurrence.startText)}
+                </time>
+                <div>
+                  <strong>{occurrence.summary}</strong>
+                  <span>
+                    {occurrence.timezone}
+                    {occurrence.allDay ? " · all day" : ""}
+                  </span>
+                </div>
+              </article>
+            ))}
+          </div>
+          <details open={conflicts.length > 0}>
+            <summary>Conflicts ({conflicts.length})</summary>
+            <ul className="diagnostics">
+              {conflicts.map((conflict, index) => (
+                <li key={`${conflict.left.uid}-${conflict.right.uid}-${index}`}>
+                  {conflict.kind}: {conflict.left.summary} ↔{" "}
+                  {conflict.right.summary} at{" "}
+                  {formatInstant(
+                    conflict.right.startMs,
+                    conflict.right.startText,
+                  )}
+                </li>
+              ))}
+            </ul>
+          </details>
+        </section>
+      )}
+
+      {tab === "merge" && (
+        <section className="panel workspace">
+          <div className="panel-heading">
+            <div>
+              <p className="eyebrow">UID + RECURRENCE-ID identity</p>
+              <h2>Merge and deduplicate</h2>
+            </div>
+            <button type="button" className="primary" onClick={merge}>
+              Merge calendars
+            </button>
+          </div>
+          <div className="merge-grid">
+            <label className="field">
+              <span>Calendar A</span>
+              <textarea
+                aria-label="Calendar A"
+                value={mergeLeft}
+                onChange={(event) => setMergeLeft(event.target.value)}
+              />
+            </label>
+            <label className="field">
+              <span>Calendar B</span>
+              <textarea
+                aria-label="Calendar B"
+                value={mergeRight}
+                onChange={(event) => setMergeRight(event.target.value)}
+              />
+            </label>
+            <label className="field">
+              <span>Canonical merged output</span>
+              <textarea
+                aria-label="Merged calendar"
+                value={mergeOutput}
+                readOnly
+                placeholder="Merge output appears here."
+              />
+            </label>
+          </div>
+          {mergeInfo && <p className="notice">{mergeInfo}</p>}
+          <div className="actions">
+            <button
+              type="button"
+              disabled={!mergeOutput}
+              onClick={() => downloadIcs(mergeOutput, "merged-calendar.ics")}
+            >
+              Download merged .ics
+            </button>
+            <button
+              type="button"
+              disabled={!mergeOutput}
+              onClick={() => {
+                if (!mergeOutput) return;
+                setSource(mergeOutput);
+                accept(mergeOutput);
+                setTab("inspect");
+              }}
+            >
+              Inspect merged output
+            </button>
+          </div>
+        </section>
+      )}
+
+      {tab === "timezone" && (
+        <section className="panel workspace">
+          <div className="panel-heading">
+            <div>
+              <p className="eyebrow">Browser IANA database</p>
+              <h2>Timezone and DST diagnostics</h2>
+            </div>
+          </div>
+          <div className="control-row timezone-controls">
+            <label className="field">
+              <span>IANA timezone</span>
+              <input
+                value={zone}
+                onChange={(event) => setZone(event.target.value)}
+                placeholder="Europe/Berlin"
+              />
+            </label>
+            <label className="field">
+              <span>Year</span>
+              <input
+                type="number"
+                min="1970"
+                max="2100"
+                value={zoneYear}
+                onChange={(event) => setZoneYear(Number(event.target.value))}
+              />
+            </label>
+            <label className="field">
+              <span>Local wall time</span>
+              <input
+                type="datetime-local"
+                step="1"
+                value={localTime}
+                onChange={(event) => setLocalTime(event.target.value)}
+              />
+            </label>
+            <button type="button" className="primary" onClick={inspectZone}>
+              Diagnose
+            </button>
+          </div>
+          <div className="facts">
+            <div>
+              <span>January offset</span>
+              <strong>{offsetLabel(zoneResult.januaryOffset)}</strong>
+            </div>
+            <div>
+              <span>July offset</span>
+              <strong>{offsetLabel(zoneResult.julyOffset)}</strong>
+            </div>
+            <div>
+              <span>Transitions</span>
+              <strong>{zoneResult.transitions.length}</strong>
+            </div>
+            <div>
+              <span>Wall-time candidates</span>
+              <strong>{candidates.length}</strong>
+            </div>
+          </div>
+          <div className="zone-grid">
+            <article>
+              <h3>Offset transitions</h3>
+              {zoneResult.transitions.length ? (
+                <ul>
+                  {zoneResult.transitions.map((transition) => (
+                    <li key={transition.at}>
+                      {transition.at}: {offsetLabel(transition.fromMinutes)} →{" "}
+                      {offsetLabel(transition.toMinutes)}
+                    </li>
+                  ))}
+                </ul>
+              ) : (
+                <p>No offset changes detected in this year.</p>
+              )}
+            </article>
+            <article>
+              <h3>Wall-time classification</h3>
+              {candidates.length === 0 && (
+                <p>
+                  Gap: this local time does not exist because clocks move
+                  forward.
+                </p>
+              )}
+              {candidates.length === 1 && (
+                <p>Unique instant: {new Date(candidates[0]!).toISOString()}</p>
+              )}
+              {candidates.length > 1 && (
+                <>
+                  <p>
+                    Overlap: this local time occurs {candidates.length} times
+                    when clocks move backward.
+                  </p>
+                  <ul>
+                    {candidates.map((candidate) => (
+                      <li key={candidate}>
+                        {new Date(candidate).toISOString()}
+                      </li>
+                    ))}
+                  </ul>
+                </>
+              )}
+            </article>
+          </div>
+          <p className="muted">
+            Results use the timezone database shipped by this browser. They can
+            differ from another runtime or from historic local law.
+          </p>
+        </section>
+      )}
+    </main>
+  );
+}

src/main.tsx

@@ -0,0 +1,18 @@
+import { StrictMode } from "react";
+import { createRoot } from "react-dom/client";
+import { App } from "./App";
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <App />
+  </StrictMode>,
+);
+
+if ("serviceWorker" in navigator && import.meta.env.PROD) {
+  window.addEventListener("load", () => {
+    const url = new URL("./sw.js", document.baseURI);
+    void navigator.serviceWorker
+      .register(url, { scope: new URL("./", document.baseURI).pathname })
+      .catch(() => undefined);
+  });
+}

src/styles.css

@@ -0,0 +1,548 @@
+:root {
+  --toolbox-background: #f6f7fb;
+  --toolbox-surface: #fff;
+  --toolbox-surface-soft: #eff1f7;
+  --toolbox-text: #202332;
+  --toolbox-muted: #656b7d;
+  --toolbox-border: #d9dce7;
+  --toolbox-accent: #5b4ec4;
+  --toolbox-accent-hover: #493caf;
+  --toolbox-accent-soft: #ece9ff;
+  --toolbox-accent-contrast: #fff;
+  --toolbox-focus: #137d75;
+  --toolbox-danger: #b42342;
+}
+* {
+  box-sizing: border-box;
+}
+html {
+  min-width: 20rem;
+  min-height: 100%;
+  background: var(--toolbox-background);
+  scrollbar-gutter: stable;
+}
+body {
+  min-width: 20rem;
+  min-height: 100vh;
+  margin: 0;
+  background: var(--toolbox-background);
+  color: var(--toolbox-text);
+  font-family: Inter, ui-sans-serif, system-ui, sans-serif;
+}
+button,
+input,
+select,
+textarea {
+  font: inherit;
+}
+button,
+.button {
+  min-height: 2.55rem;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  gap: 0.4rem;
+  padding: 0.55rem 0.8rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.65rem;
+  background: var(--toolbox-surface);
+  color: var(--toolbox-text);
+  font-weight: 720;
+  cursor: pointer;
+}
+button:hover:not(:disabled),
+.button:hover {
+  border-color: var(--toolbox-accent);
+  background: var(--toolbox-surface-soft);
+}
+:where(button, input, select, textarea, a):focus-visible {
+  outline: 3px solid color-mix(in srgb, var(--toolbox-focus) 42%, transparent);
+  outline-offset: 2px;
+}
+input,
+select,
+textarea {
+  width: 100%;
+  min-height: 2.55rem;
+  padding: 0.58rem 0.7rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.62rem;
+  background: var(--toolbox-surface);
+  color: var(--toolbox-text);
+}
+textarea {
+  min-height: 10rem;
+  resize: vertical;
+  font-family: ui-monospace, SFMono-Regular, Consolas, monospace;
+  line-height: 1.48;
+}
+.toolbox-shell__main {
+  width: min(100%, 90rem);
+  padding: clamp(0.75rem, 1.8vw, 1.5rem);
+}
+.workbench {
+  display: grid;
+  gap: 1rem;
+}
+.hero,
+.panel {
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.9rem;
+  background: var(--toolbox-surface);
+  box-shadow: 0 8px 28px rgb(30 36 70 / 4%);
+}
+.hero {
+  display: flex;
+  justify-content: space-between;
+  gap: 1rem;
+  align-items: flex-start;
+  padding: clamp(1.1rem, 3vw, 2rem);
+}
+.hero h1,
+.panel h2,
+.panel h3,
+.help-dialog h2,
+.fatal h1 {
+  margin: 0;
+  letter-spacing: -0.025em;
+}
+.hero p:not(.eyebrow) {
+  max-width: 52rem;
+  margin: 0.55rem 0 0;
+  color: var(--toolbox-muted);
+  line-height: 1.55;
+}
+.eyebrow {
+  margin: 0 0 0.3rem;
+  color: var(--toolbox-accent);
+  font-size: 0.69rem;
+  font-weight: 820;
+  letter-spacing: 0.115em;
+  text-transform: uppercase;
+}
+.privacy-pill {
+  flex: 0 0 auto;
+  padding: 0.38rem 0.62rem;
+  border-radius: 999px;
+  background: var(--toolbox-accent-soft);
+  color: var(--toolbox-accent);
+  font-size: 0.75rem;
+  font-weight: 760;
+}
+.panel {
+  padding: 1rem;
+}
+.panel-heading {
+  display: flex;
+  justify-content: space-between;
+  gap: 1rem;
+  align-items: end;
+  margin-bottom: 0.9rem;
+}
+.capability-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(min(100%, 13rem), 1fr));
+  gap: 0.75rem;
+}
+.capability-grid article {
+  padding: 0.9rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.72rem;
+  background: var(--toolbox-surface-soft);
+}
+.capability-grid p {
+  margin: 0.4rem 0 0;
+  color: var(--toolbox-muted);
+  line-height: 1.48;
+}
+.workspace-tabs {
+  display: flex;
+  gap: 0.4rem;
+  overflow-x: auto;
+  padding-bottom: 0.2rem;
+}
+.workspace-tabs button[aria-selected="true"] {
+  border-color: var(--toolbox-accent);
+  background: var(--toolbox-accent);
+  color: var(--toolbox-accent-contrast);
+}
+.grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(min(100%, 20rem), 1fr));
+  gap: 0.85rem;
+}
+.field {
+  display: grid;
+  gap: 0.35rem;
+}
+.field > span {
+  font-size: 0.76rem;
+  font-weight: 750;
+}
+.muted {
+  color: var(--toolbox-muted);
+}
+.result {
+  padding: 0.8rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.68rem;
+  background: var(--toolbox-surface-soft);
+  overflow-wrap: anywhere;
+}
+.loading,
+.fatal {
+  width: min(100% - 2rem, 60rem);
+  margin: 2rem auto;
+  padding: 1rem;
+}
+.help-dialog {
+  width: min(36rem, calc(100% - 2rem));
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.9rem;
+  background: var(--toolbox-surface);
+  color: var(--toolbox-text);
+}
+.help-dialog::backdrop {
+  background: rgb(20 24 45 / 55%);
+}
+.dialog-heading {
+  display: flex;
+  justify-content: space-between;
+  gap: 1rem;
+  align-items: start;
+}
+.workspace {
+  display: grid;
+  gap: 0.85rem;
+}
+.editor-grid {
+  display: grid;
+  grid-template-columns: repeat(2, minmax(0, 1fr));
+  gap: 1rem;
+}
+.editor-grid textarea {
+  min-height: 22rem;
+}
+.encoding-row {
+  display: grid;
+  grid-template-columns: minmax(10rem, 16rem) minmax(12rem, 1fr);
+  gap: 0.7rem;
+  align-items: end;
+}
+.actions {
+  display: flex;
+  gap: 0.5rem;
+  flex-wrap: wrap;
+}
+.primary {
+  border-color: var(--toolbox-accent);
+  background: var(--toolbox-accent);
+  color: var(--toolbox-accent-contrast);
+}
+.file-button {
+  position: relative;
+  overflow: hidden;
+}
+.file-button input {
+  position: absolute;
+  inset: 0;
+  opacity: 0;
+  cursor: pointer;
+}
+.check {
+  display: flex;
+  gap: 0.5rem;
+  align-items: center;
+  min-height: 2.55rem;
+}
+.check input {
+  width: auto;
+  min-height: auto;
+}
+.inventory {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(7rem, 1fr));
+  gap: 0.45rem;
+  margin: 0;
+}
+.inventory div {
+  padding: 0.55rem;
+  border-radius: 0.55rem;
+  background: var(--toolbox-surface-soft);
+}
+.inventory dt {
+  color: var(--toolbox-muted);
+  font-size: 0.66rem;
+  font-weight: 750;
+}
+.inventory dd {
+  margin: 0.15rem 0 0;
+}
+.steps {
+  display: grid;
+  gap: 0.55rem;
+  margin: 0;
+  padding: 0;
+  list-style: none;
+}
+.steps li {
+  display: grid;
+  grid-template-columns: minmax(12rem, 0.8fr) minmax(12rem, 1.4fr) auto;
+  gap: 0.6rem;
+  align-items: center;
+  padding: 0.6rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.65rem;
+}
+.step-actions {
+  display: flex;
+  gap: 0.25rem;
+}
+.step-actions button {
+  min-width: 2.55rem;
+  padding: 0.4rem;
+}
+.warning,
+.error,
+.notice {
+  margin: 0;
+  padding: 0.7rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.65rem;
+  line-height: 1.5;
+}
+.warning {
+  border-color: #d9a72e;
+  background: #fff8df;
+  color: #725000;
+}
+.error {
+  border-color: var(--toolbox-danger);
+  color: var(--toolbox-danger);
+}
+.notice {
+  background: var(--toolbox-surface-soft);
+}
+table {
+  width: 100%;
+  border-collapse: collapse;
+}
+th,
+td {
+  padding: 0.55rem;
+  border-bottom: 1px solid var(--toolbox-border);
+  text-align: left;
+}
+th {
+  color: var(--toolbox-muted);
+  font-size: 0.7rem;
+  text-transform: uppercase;
+}
+details {
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.65rem;
+}
+summary {
+  padding: 0.65rem;
+  cursor: pointer;
+  font-weight: 750;
+}
+.recipe {
+  display: grid;
+  gap: 0.6rem;
+  padding: 0 0.65rem 0.65rem;
+}
+.alert,
+.warning,
+.notice {
+  margin: 0;
+  padding: 0.75rem 0.9rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.68rem;
+}
+.alert {
+  border-color: var(--toolbox-danger);
+  color: var(--toolbox-danger);
+  background: color-mix(
+    in srgb,
+    var(--toolbox-danger) 7%,
+    var(--toolbox-surface)
+  );
+}
+.warning {
+  border-color: #c18a12;
+  background: color-mix(in srgb, #e0a423 12%, var(--toolbox-surface));
+}
+.notice {
+  background: var(--toolbox-surface-soft);
+}
+.split-layout {
+  display: grid;
+  grid-template-columns: minmax(20rem, 0.9fr) minmax(0, 1.25fr);
+  gap: 1rem;
+  align-items: start;
+}
+.split-layout textarea {
+  min-height: 29rem;
+}
+.compact {
+  width: auto;
+}
+.facts {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(8rem, 1fr));
+  gap: 0.6rem;
+}
+.facts > div {
+  display: grid;
+  gap: 0.25rem;
+  padding: 0.75rem;
+  border-radius: 0.65rem;
+  background: var(--toolbox-surface-soft);
+}
+.facts span,
+.metadata dt,
+tbody td span,
+.timeline span {
+  color: var(--toolbox-muted);
+  font-size: 0.75rem;
+}
+.facts strong {
+  font-size: 1.15rem;
+}
+.metadata {
+  display: grid;
+  gap: 0.45rem;
+  margin: 0;
+}
+.metadata > div {
+  display: grid;
+  grid-template-columns: minmax(7rem, 0.3fr) minmax(0, 1fr);
+  gap: 0.6rem;
+}
+.metadata dd {
+  margin: 0;
+  overflow-wrap: anywhere;
+}
+.table-scroll {
+  max-height: 28rem;
+  overflow: auto;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.65rem;
+}
+table {
+  width: 100%;
+  border-collapse: collapse;
+}
+th,
+td {
+  padding: 0.6rem;
+  border-bottom: 1px solid var(--toolbox-border);
+  text-align: left;
+  vertical-align: top;
+}
+th {
+  position: sticky;
+  top: 0;
+  z-index: 1;
+  background: var(--toolbox-surface);
+  color: var(--toolbox-muted);
+  font-size: 0.69rem;
+  text-transform: uppercase;
+}
+tbody td:first-child {
+  display: grid;
+  gap: 0.2rem;
+}
+.diagnostics,
+.notice-list {
+  max-height: 18rem;
+  overflow: auto;
+  line-height: 1.5;
+}
+.diagnostics [data-severity="error"] {
+  color: var(--toolbox-danger);
+}
+.control-row {
+  display: grid;
+  grid-template-columns: repeat(2, minmax(9rem, 14rem)) auto;
+  gap: 0.7rem;
+  align-items: end;
+}
+.timezone-controls {
+  grid-template-columns:
+    minmax(12rem, 1fr) minmax(7rem, 0.35fr) minmax(14rem, 0.8fr)
+    auto;
+}
+.timeline {
+  display: grid;
+  gap: 0.45rem;
+  max-height: 38rem;
+  overflow: auto;
+}
+.timeline article {
+  display: grid;
+  grid-template-columns: minmax(13rem, 0.55fr) minmax(0, 1fr);
+  gap: 0.8rem;
+  padding: 0.65rem;
+  border-left: 4px solid var(--toolbox-accent);
+  border-radius: 0.45rem;
+  background: var(--toolbox-surface-soft);
+}
+.timeline div {
+  display: grid;
+  gap: 0.2rem;
+}
+.merge-grid {
+  display: grid;
+  grid-template-columns: repeat(3, minmax(0, 1fr));
+  gap: 0.8rem;
+}
+.merge-grid textarea {
+  min-height: 27rem;
+}
+.zone-grid {
+  display: grid;
+  grid-template-columns: repeat(2, minmax(0, 1fr));
+  gap: 0.8rem;
+}
+.zone-grid article {
+  padding: 0.9rem;
+  border: 1px solid var(--toolbox-border);
+  border-radius: 0.7rem;
+  background: var(--toolbox-surface-soft);
+}
+.zone-grid h3 {
+  margin-top: 0;
+}
+@media (max-width: 62rem) {
+  .editor-grid {
+    grid-template-columns: 1fr;
+  }
+  .split-layout,
+  .merge-grid,
+  .zone-grid {
+    grid-template-columns: 1fr;
+  }
+  .timezone-controls {
+    grid-template-columns: repeat(2, minmax(0, 1fr));
+  }
+}
+@media (max-width: 48rem) {
+  .steps li {
+    grid-template-columns: 1fr;
+  }
+}
+@media (max-width: 42rem) {
+  .hero {
+    flex-direction: column;
+  }
+  .privacy-pill {
+    order: -1;
+  }
+  .control-row,
+  .timezone-controls,
+  .timeline article {
+    grid-template-columns: 1fr;
+  }
+}

src/test/setup.ts

@@ -0,0 +1,8 @@
+import "@testing-library/jest-dom/vitest";
+import { afterEach } from "vitest";
+import { cleanup } from "@testing-library/react";
+
+afterEach(() => {
+  cleanup();
+  localStorage.clear();
+});

src/toolbox/manifest.source.json

@@ -0,0 +1,41 @@
+{
+  "$schema": "https://git.add-ideas.de/lotobo/toolbox-sdk/raw/branch/main/schemas/toolbox-app.v1.schema.json",
+  "schemaVersion": 1,
+  "id": "de.add-ideas.calendar-tools",
+  "name": "Calendar Tools",
+  "version": "0.1.0",
+  "description": "Inspect, repair, and reconcile calendars locally in the browser.",
+  "entry": "./",
+  "icon": "./favicon.svg",
+  "categories": ["productivity", "data", "time"],
+  "tags": ["calendar", "ics", "icalendar", "rrule", "timezone", "dst", "merge"],
+  "integration": {
+    "contextVersion": 1,
+    "launchModes": ["navigate", "new-tab"],
+    "embedding": "unsupported"
+  },
+  "requirements": {
+    "secureContext": false,
+    "workers": false,
+    "indexedDb": false,
+    "crossOriginIsolated": false,
+    "topLevelContext": false
+  },
+  "privacy": {
+    "processing": "local",
+    "fileUploads": true,
+    "telemetry": false,
+    "label": "Inputs stay in this browser; nothing is uploaded."
+  },
+  "source": {
+    "repository": "https://git.add-ideas.de/lotobo/calendar-tools",
+    "license": "GPL-3.0-or-later"
+  },
+  "actions": [
+    {
+      "id": "source",
+      "label": "Source",
+      "url": "https://git.add-ideas.de/lotobo/calendar-tools"
+    }
+  ]
+}

src/toolbox/manifest.ts

@@ -0,0 +1,4 @@
+import { defineToolboxApp, parseToolboxApp } from "@add-ideas/toolbox-contract";
+import source from "./manifest.source.json";
+
+export const manifest = defineToolboxApp(parseToolboxApp(source));

src/version.ts

@@ -0,0 +1 @@
+export const APP_VERSION = "0.1.0";

src/vite-env.d.ts

@@ -0,0 +1 @@
+/// <reference types="vite/client" />

tests/browser/app.spec.ts

@@ -0,0 +1,62 @@
+import { expect, test, type Page } from "@playwright/test";
+
+const ORIGIN = "http://127.0.0.1:4173";
+async function localOnly(page: Page) {
+  const external: string[] = [];
+  await page.route("**/*", async (route) => {
+    const url = new URL(route.request().url());
+    if (url.origin !== ORIGIN) {
+      external.push(url.href);
+      await route.abort();
+    } else await route.continue();
+  });
+  return external;
+}
+
+test("runs from a nested path without external requests", async ({ page }) => {
+  const errors: string[] = [];
+  page.on("pageerror", (error) => errors.push(error.message));
+  page.on("console", (message) => {
+    if (message.type() === "error") errors.push(message.text());
+  });
+  const external = await localOnly(page);
+  await page.goto("/deep/nested/calendar/");
+  await expect(
+    page.getByRole("banner").getByRole("heading", { name: "Calendar Tools" }),
+  ).toBeVisible();
+  expect(external).toEqual([]);
+  expect(errors).toEqual([]);
+});
+
+test("keeps prior inspection on failure and diagnoses a DST overlap", async ({
+  page,
+}) => {
+  const external = await localOnly(page);
+  await page.goto("/deep/nested/calendar/");
+  await expect(page.getByText("Weekly review", { exact: true })).toBeVisible();
+  await page.getByLabel("iCalendar source").fill("broken");
+  await page.getByRole("button", { name: "Inspect calendar" }).click();
+  await expect(page.getByRole("alert")).toBeVisible();
+  await page.getByRole("tab", { name: "Timezone" }).click();
+  await page.getByRole("button", { name: "Diagnose" }).click();
+  await expect(
+    page.getByText(/Overlap: this local time occurs 2 times/u),
+  ).toBeVisible();
+  expect(external).toEqual([]);
+});
+
+test("serves release identity and hardened headers", async ({ request }) => {
+  const index = await request.get("/deep/nested/calendar/");
+  expect(index.ok()).toBe(true);
+  expect(index.headers()["content-security-policy"]).toContain(
+    "connect-src 'self'",
+  );
+  expect(await index.text()).not.toMatch(/\b(?:src|href)=["']\//u);
+  const manifest = await request.get("/deep/nested/calendar/toolbox-app.json");
+  await expect(manifest.json()).resolves.toMatchObject({
+    id: "de.add-ideas.calendar-tools",
+    version: "0.1.0",
+    entry: "./",
+    privacy: { processing: "local", telemetry: false },
+  });
+});

tests/calendar/model.test.ts

@@ -0,0 +1,87 @@
+import { describe, expect, it } from "vitest";
+import {
+  canonicalRepair,
+  findConflicts,
+  localTimeCandidates,
+  mergeCalendars,
+  parseCalendar,
+  previewOccurrences,
+  timezoneDiagnostics,
+} from "../../src/calendar/model";
+
+const recurring = `BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//test//EN\r\nBEGIN:VEVENT\r\nUID:a\r\nDTSTAMP:20260101T000000Z\r\nDTSTART:20260901T100000Z\r\nDTEND:20260901T110000Z\r\nRRULE:FREQ=DAILY;COUNT=3\r\nSUMMARY:A\r\nEND:VEVENT\r\nBEGIN:VEVENT\r\nUID:b\r\nDTSTART:20260901T103000Z\r\nDTEND:20260901T120000Z\r\nSUMMARY:B\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n`;
+
+describe("calendar model", () => {
+  it("parses and expands recurrence under a bound", () => {
+    const document = parseCalendar(recurring);
+    expect(document.events).toHaveLength(2);
+    const preview = previewOccurrences(document, 10, 30);
+    expect(preview.occurrences.filter((item) => item.uid === "a")).toHaveLength(
+      3,
+    );
+    expect(findConflicts(preview.occurrences)).toEqual(
+      expect.arrayContaining([expect.objectContaining({ kind: "overlap" })]),
+    );
+  });
+
+  it("canonicalizes line endings and supplies required metadata", () => {
+    const input = `BEGIN:VCALENDAR\nBEGIN:VEVENT\nUID:x\nDTSTART:20260901T100000Z\nDTEND:20260901T110000Z\nEND:VEVENT\nEND:VCALENDAR\n`;
+    const repaired = canonicalRepair(input);
+    expect(repaired.source).toContain("VERSION:2.0\r\n");
+    expect(repaired.source).toContain("PRODID:");
+    expect(repaired.source).not.toMatch(/(?<!\r)\n/u);
+  });
+
+  it("merges event identity using sequence then stamp", () => {
+    const newer = recurring.replace(
+      "SUMMARY:A",
+      "SEQUENCE:2\r\nSUMMARY:Updated A",
+    );
+    const merged = mergeCalendars([
+      parseCalendar(recurring),
+      parseCalendar(newer),
+    ]);
+    expect(merged.deduplicated).toBe(2);
+    expect(merged.source).toContain("SUMMARY:Updated A");
+    expect(parseCalendar(merged.source).events).toHaveLength(2);
+  });
+
+  it("never deduplicates separate UID-less events", () => {
+    const uidless = `BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//test//EN\r\nBEGIN:VEVENT\r\nDTSTART:20260901T100000Z\r\nDTEND:20260901T110000Z\r\nSUMMARY:No identity\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n`;
+    const merged = mergeCalendars([
+      parseCalendar(uidless),
+      parseCalendar(uidless),
+    ]);
+    expect(merged.deduplicated).toBe(0);
+    expect(parseCalendar(merged.source).events).toHaveLength(2);
+    expect(merged.diagnostics.map((item) => item.message).join(" ")).toMatch(
+      /UID-less/u,
+    );
+  });
+
+  it("uses embedded VTIMEZONE observances for custom TZIDs", () => {
+    const custom = `BEGIN:VCALENDAR\r\nVERSION:2.0\r\nPRODID:-//test//EN\r\nBEGIN:VTIMEZONE\r\nTZID:Custom/Test\r\nBEGIN:STANDARD\r\nDTSTART:19700101T000000\r\nTZOFFSETFROM:+0230\r\nTZOFFSETTO:+0230\r\nTZNAME:CST\r\nEND:STANDARD\r\nEND:VTIMEZONE\r\nBEGIN:VEVENT\r\nUID:custom\r\nDTSTART;TZID=Custom/Test:20260901T100000\r\nDTEND;TZID=Custom/Test:20260901T110000\r\nSUMMARY:Custom zone\r\nEND:VEVENT\r\nEND:VCALENDAR\r\n`;
+    const document = parseCalendar(custom);
+    expect(document.timezones[0]?.tzid).toBe("Custom/Test");
+    expect(document.events[0]?.startMs).toBe(
+      Date.parse("2026-09-01T07:30:00.000Z"),
+    );
+  });
+
+  it("classifies DST overlaps and reports yearly transitions", () => {
+    const candidates = localTimeCandidates("2026-10-25T02:30", "Europe/Berlin");
+    expect(candidates).toHaveLength(2);
+    expect(timezoneDiagnostics("Europe/Berlin", 2026).transitions).toHaveLength(
+      2,
+    );
+  });
+
+  it("rejects unbounded and malformed inputs", () => {
+    expect(() => previewOccurrences(parseCalendar(recurring), 501, 30)).toThrow(
+      /1–500/u,
+    );
+    expect(() => parseCalendar("not a calendar")).toThrow(
+      /parse failed|root component/u,
+    );
+  });
+});

tests/components/app.test.tsx

@@ -0,0 +1,24 @@
+import { render, screen } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { describe, expect, it, vi } from "vitest";
+import { App } from "../../src/App";
+
+describe("Calendar Tools", () => {
+  it("renders the local workbench and bounded recurrence view", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => new Response("Not found", { status: 404 })),
+    );
+    render(<App />);
+    expect(
+      await screen.findByRole("heading", { name: "Calendar Tools" }),
+    ).toBeVisible();
+    await userEvent.click(
+      await screen.findByRole("tab", { name: "Recurrence" }),
+    );
+    expect(
+      screen.getByRole("heading", { name: "Occurrence and conflict preview" }),
+    ).toBeVisible();
+    expect(screen.getByText("Browser-local")).toBeVisible();
+  });
+});

tsconfig.app.json

@@ -0,0 +1,27 @@
+{
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.app.tsbuildinfo",
+    "target": "ES2023",
+    "lib": ["ES2023", "DOM", "DOM.Iterable"],
+    "module": "ESNext",
+    "types": ["vite/client"],
+    "allowArbitraryExtensions": true,
+    "skipLibCheck": true,
+    "moduleResolution": "Bundler",
+    "allowImportingTsExtensions": true,
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+    "jsx": "react-jsx",
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "erasableSyntaxOnly": true,
+    "noFallthroughCasesInSwitch": true,
+    "strict": true,
+    "noUncheckedIndexedAccess": true,
+    "useDefineForClassFields": true,
+    "resolveJsonModule": true,
+    "isolatedModules": true
+  },
+  "include": ["src", "tests"]
+}

tsconfig.json

@@ -0,0 +1,11 @@
+{
+  "files": [],
+  "references": [
+    {
+      "path": "./tsconfig.app.json"
+    },
+    {
+      "path": "./tsconfig.node.json"
+    }
+  ]
+}

tsconfig.node.json

@@ -0,0 +1,24 @@
+{
+  "compilerOptions": {
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.node.tsbuildinfo",
+    "target": "ES2023",
+    "lib": ["ES2023"],
+    "module": "ESNext",
+    "types": ["node"],
+    "skipLibCheck": true,
+    "moduleResolution": "Bundler",
+    "allowImportingTsExtensions": true,
+    "verbatimModuleSyntax": true,
+    "moduleDetection": "force",
+    "noEmit": true,
+    "strict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true
+  },
+  "include": [
+    "vite.config.ts",
+    "playwright.config.ts",
+    "eslint.config.mjs",
+    "scripts/**/*.mjs"
+  ]
+}

vite.config.ts

@@ -0,0 +1,16 @@
+/// <reference types="vitest/config" />
+import { defineConfig } from "vite";
+import react from "@vitejs/plugin-react";
+
+export default defineConfig({
+  base: "./",
+  plugins: [react()],
+  test: {
+    environment: "jsdom",
+    setupFiles: "./src/test/setup.ts",
+    css: true,
+    maxWorkers: 2,
+    restoreMocks: true,
+    exclude: ["tests/browser/**", "node_modules/**", "dist/**"],
+  },
+});