pr-description.md

Description

Implements offline-first data synchronization and local caching for the mobile web app (PWA) to enable full functionality with intermittent or no connectivity.

Changes Made

Core Infrastructure

• IndexedDB Layer: Persistent local storage using idb library
• Sync Service: Manages online/offline state and background sync
• Action Queue System: Queues offline writes with replay on reconnection

React Integration

• New hooks: useOfflineSyncInit, useSyncStatus, useIsOnline, useQueueAction
• Enhanced useGroups with offline cache fallback

UI Components

• OfflineIndicator: Shows connection/sync status in header
• StaleDataBanner: Warns about cached/stale data

Service Worker Enhancements

• Network-first API caching with 5-minute TTL
• Background Sync API support

Documentation

• Comprehensive guide in docs/offline-first-sync.md

Acceptance Criteria

• App remains usable read-only with no connection
• Local cache for groups, members, history
• Sync on foreground and background interval
• Queue write actions while offline
• Replay queue on reconnect in order
• No duplicate submission
• Show stale data and offline indicators

Dependencies Added

• idb@^8.0.0: IndexedDB wrapper library
• workbox-window@^7.0.0: Service worker utilities

Closes #1005