Implement offline sync with WorkManager and DataStore for Android

Summary

Comprehensive offline sync service using Android best practices with WorkManager and DataStore

Changes

• Added OfflineSyncService.kt with full offline sync implementation (539 lines)
• Auto-sync on network connection (ConnectivityManager)
• Periodic WorkManager background sync (15 min intervals)
• Sync queue with exponential backoff retry
• DataStore Preferences for fast persistence
• StateFlow for reactive UI updates
• Conflict detection and resolution

Architecture

• WorkManager: Reliable background work with constraints
• DataStore: Modern replacement for SharedPreferences
• ConnectivityManager: Network state monitoring with callbacks
• StateFlow: Reactive sync status updates for Compose UI
• Coroutines: Structured concurrency for async operations

Data Synchronized

• User profile from user-service (java-microservices)
• Recent content from content-service (java-microservices)
• User favorites
• Pending operations queue

Conflict Resolution Strategies

• LOCAL_WINS: Force local changes
• REMOTE_WINS: Accept remote state
• MERGE: Attempt automatic merge
• USER_CHOICE: Prompt user for decision

Integration Points

• Depends on user-service from java-microservices group
• Depends on content-service from java-microservices group
• SyncWorker for WorkManager background execution

Testing

• Network state transitions (WiFi/cellular/offline)
• WorkManager periodic sync execution
• DataStore persistence across app restarts
• Conflict resolution all strategies
• Manual sync (pull-to-refresh)
• Background sync constraints

Related Issues

Closes #3 (closed)

Cross-group dependencies

• Depends on user-service (java-microservices group)
• Depends on content-service (java-microservices group)
• Coordinates with ios-app (mobile-section