[FEATURE]: Add offline announcement draft queue with background sync recovery

[Hanu2908]

Description

ClassHub is an academic PWA designed for mobile-first campus environments with intermittent Wi-Fi and mobile connectivity. Currently, if a Class Representative or Teacher attempts to publish an announcement while offline or during a network drop, the request fails.

We need to extend our existing IndexedDB offline action queue (src/lib/offlineDb.ts) to store pending announcement drafts locally and automatically replay them when network connectivity is restored.

Relevant files

• src/lib/offlineDb.ts (IndexedDB schema and action queue helpers)
• src/lib/offlineSync.ts (Network listener and background action playback)
• src/pages/app/AnnouncementsPage.tsx (Announcement composer)
• tests/unit/offlineSync.test.ts (Unit test suite for offline queueing and playback)

Requirements and acceptance criteria

1. IndexedDB action extension:

   • Extend OfflineAction in src/lib/offlineDb.ts to support type: 'announcement'.
   • Ensure the action stores payload data: title, body, priority, deadline, targetBatch, sectionId, and authorId.

2. Offline composer behavior:

   • When submitting an announcement while !navigator.onLine or upon a fetch failure, enqueue the action via enqueueAction('announcement', payload).
   • Display a notification/indicator: "Saved offline — will publish automatically when you reconnect".
   • Optimistically prepend the draft into the announcements list with a subtle "Queued (Offline)" badge.

3. Reconnection & playback:

   • In src/lib/offlineSync.ts, update playbackOfflineActionsClient to handle 'announcement' actions by inserting them into Supabase via supabase.from('announcements').insert(...).
   • Upon successful server response, dequeue the action from IndexedDB and invalidate the React Query cache ['announcements', sectionId].

4. Testing:

   • Add unit tests in tests/unit/offlineSync.test.ts covering:
     • Enqueueing an announcement payload when offline.
     • Playback execution upon online event.
     • Successful dequeueing and retry on failure.
   • All tests must pass with npm test.

Testing locally

npm test tests/unit/offlineSync.test.ts