Refactor the messaging system to improve synchronization efficiency and state consistency. - Implement incremental message synchronization using sequence numbers (seq) to reduce payload size. - Add `markAllAsRead` batch API support to optimize read receipt processing. - Introduce atomic state updates in `useMessageStore` to prevent inconsistent UI rendering during unread count changes. - Implement notification deduplication in `WSMessageHandler` to prevent duplicate unread increments during reconnection. - Optimize `MessageSyncService` to prioritize incremental sync over full snapshots when local data exists. - Refactor `ReadReceiptManager` to use optimistic updates with proper rollback mechanisms. - Fix minor UI issues in `SettingsScreen` and `PrivacySettingsScreen` related to theme picker z-index and style application.