Add offline mode and data syncing

Implement offline functionality with background sync.

Features:

• Local data persistence (Core Data/Realm)
• Offline queue for API requests
• Background sync when online
• Conflict resolution strategy
• Sync status indicators
• Optimistic UI updates

Sync Strategy:

• Auto-sync when app becomes active
• Background sync every 15 minutes
• Manual sync option
• WiFi-only sync option in settings

Data to Cache:

• User profile
• Recent content
• Favorites/bookmarks
• Draft content