Add PWA offline support with service worker caching

[Smartdevs17]

Description

Implement comprehensive offline support via service worker caching strategy, allowing the dashboard to function with limited capability when offline and sync when connectivity returns.

Acceptance Criteria

• Service worker with cache-first strategy for static assets
• Network-first strategy for API data with offline fallback
• Background sync for queued operations (retry on reconnect)
• Offline indicator with connection status monitoring
• Cache invalidation on app update
• Offline-capable payment queue

Technical Scope

• frontend/service-worker.ts
• frontend/lib/offline.tsx
• frontend/components/layout/Header.tsx
• frontend/app/service-worker.js/route.ts
• Edge: stale cache serving, storage quota exceeded, concurrent sync conflicts