feat: implement PWA offline fallback and improved service worker handling

[saidai-bhuvanesh]

Fixes #6100

Phase 1: PWA Reliability & Offline Fallback

• Added a branded offline fallback UI in app/offline/page.tsx (OfflineFallback component).
• Configured the service worker (app/sw.ts) to cache the /offline fallback page during installation.
• Updated the service worker fetch handler to gracefully serve the cached /offline fallback page on navigation failure.
• Implemented corresponding tests in tests/pwa/offline-page.test.tsx.

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