feat: export parser sessions from the browser

src/App.test.tsx

@@ -1,6 +1,6 @@
 import { fireEvent, render, screen, within } from '@testing-library/react';
 import userEvent from '@testing-library/user-event';
-import { describe, expect, it } from 'vitest';
+import { describe, expect, it, vi } from 'vitest';
 
 import { OneNoteApplication } from './App.js';
 import type {
@@ -105,6 +105,17 @@ class SuccessfulParserWorker implements WorkerPort {
             diagnostics: [],
           },
         });
+      } else if (message.type === 'export-session') {
+        this.respond({
+          type: 'export-artifact',
+          requestId: message.requestId,
+          sessionId: message.sessionId,
+          artifact: {
+            filename: 'Project notes.zip',
+            mediaType: 'application/zip',
+            bytes: Uint8Array.of(0x50, 0x4b, 3, 4).buffer,
+          },
+        });
       }
     });
   }
@@ -183,6 +194,43 @@ describe('OneNoteApplication', () => {
     ).toBeInTheDocument();
   });
 
+  it('downloads a worker-built export after explicit user action', async () => {
+    const user = userEvent.setup();
+    const worker = new SuccessfulParserWorker();
+    const createObjectUrl = vi
+      .spyOn(URL, 'createObjectURL')
+      .mockReturnValue('blob:local-export');
+    const click = vi
+      .spyOn(HTMLAnchorElement.prototype, 'click')
+      .mockImplementation(() => undefined);
+    render(
+      <OneNoteApplication
+        createWorkerClient={() => new OneNoteWorkerClient(() => worker)}
+      />
+    );
+
+    await user.upload(
+      screen.getByLabelText('Choose OneNote files'),
+      new File([new Uint8Array([1, 2, 3])], 'Section.one')
+    );
+    await screen.findByRole('heading', { name: 'First meeting' });
+    await user.click(screen.getByRole('button', { name: 'Create export' }));
+
+    expect(
+      await screen.findByText('The export download is ready.')
+    ).toBeVisible();
+    expect(worker.lastMessage).toEqual({
+      type: 'export-session',
+      requestId: 'export-3',
+      sessionId: 'session-one',
+      format: 'static-zip',
+    });
+    expect(createObjectUrl).toHaveBeenCalledWith(
+      expect.objectContaining({ type: 'application/zip' })
+    );
+    expect(click).toHaveBeenCalledOnce();
+  });
+
   it('opens a dropped OneNote package through the local worker', async () => {
     const notebook: OneNotePackageDto = {
       format: 'onepkg',