Add offline support with service worker caching

[uboho-prog]

close #584

PR: Implement Offline Mode Support for Learning Content

Summary

This PR adds offline-first support to the frontend learning experience. It introduces a service worker for app shell caching, persistent API response caching for offline read access, and a queued request sync mechanism for offline write operations.

Key Changes

• Added public/sw.js to cache navigation, assets, and provide an offline fallback.
• Extended API caching in src/lib/api-cache.ts to persist GET responses in localStorage and serve cached content when offline.
• Added src/lib/offline-sync.ts for queuing POST/PUT/PATCH/DELETE requests in IndexedDB and flushing them when the app regains connectivity.
• Integrated offline sync at runtime with src/components/OfflineSyncHandler.tsx.
• Updated src/app/layout.tsx to include offline notifications and sync handler components.
• Added tests for API cache behavior and offline sync queue handling.
• Updated frontend documentation to describe offline mode behavior.

Testing

• npm test in the frontend package should pass.
• Manual validation:
  1. Load the app while online and navigate learning content.
  2. Confirm offline-ready notification appears after service worker activation.
  3. Go offline and refresh; previously viewed content should still display.
  4. Perform a write action while offline and verify it syncs automatically when back online.

Notes

• This implementation focuses on learning content availability and automatic sync for offline write operations.
• The service worker is scoped to the app and caches both shell assets and navigation responses for better offline reliability.

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