chore(deps): upgrade Expo SDK from 55 to 56

Migrate from @react-navigation/native to expo-router navigation hooks across all screens.
Add polyfills module and apply LiveKit VideoView optional loading for Expo Go compatibility.
Improve background sync to skip when user is not logged in.
Enhance fetchUnreadCount to return typed totalUnread and systemUnread values.

src/services/background/backgroundService.ts

@@ -50,6 +50,11 @@ TaskManager.defineTask(BACKGROUND_SYNC_TASK, async () => {
  * 执行消息同步
  */
 async function syncMessages(): Promise<void> {
+  // 未登录时跳过同步，避免报"登录已过期"错误
+  const token = await api.getToken();
+  if (!token) {
+    return;
+  }
   try {
     // 同步会话未读数
     const unreadData = await messageService.getUnreadCount();

src/services/livekit/LiveKitService.ts

@@ -1,3 +1,4 @@
+import '../../polyfills';
 import { Room, RoomEvent, Track, ConnectionState, LocalParticipant, RemoteParticipant, RemoteTrackPublication, TrackPublication, Participant as LKParticipant } from 'livekit-client';
 
 export interface LiveKitServiceConfig {