feat(calendar): expose an accessible calendar picker

webui/tests/calendar-picker.test.ts

@@ -0,0 +1,37 @@
+import {
+  calendarPickerOptions,
+  calendarPickerTenantId,
+  type CalendarPickerOption
+} from "../src/features/calendar/calendarPickerLogic";
+
+function assert(condition: unknown, message: string): void {
+  if (!condition) throw new Error(message);
+}
+
+function calendar(id: string, name: string, isDefault = false): CalendarPickerOption {
+  return {
+    id,
+    name,
+    is_default: isDefault
+  };
+}
+
+const input = [
+  calendar("z", "Zulu"),
+  calendar("b", "Bravo", true),
+  calendar("a", "alpha")
+];
+const ordered = calendarPickerOptions(input);
+
+assert(ordered.map((item) => item.id).join(",") === "b,a,z", "default calendar should lead, followed by names");
+assert(input.map((item) => item.id).join(",") === "z,b,a", "picker sorting must not mutate API data");
+assert(
+  calendarPickerTenantId({ tenant: { id: "fallback" }, active_tenant: { id: "active" } }) === "active",
+  "active tenant should partition picker requests"
+);
+assert(
+  calendarPickerTenantId({ tenant: { id: "fallback" } }) === "fallback",
+  "legacy tenant should remain a supported fallback"
+);
+
+console.log("Calendar picker behavior checks passed.");