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/components/call/CallScreen.tsx

@@ -9,7 +9,13 @@ import {
 } from 'react-native';
 import { MaterialCommunityIcons } from '@expo/vector-icons';
 import { Track, LocalVideoTrack, RemoteVideoTrack } from 'livekit-client';
-import { VideoView } from '@livekit/react-native';
+
+let VideoView: React.ComponentType<any> | null = null;
+try {
+  VideoView = require('@livekit/react-native').VideoView;
+} catch {
+  // WebRTC native module not available (e.g. Expo Go)
+}
 
 type VideoTrack = LocalVideoTrack | RemoteVideoTrack;
 import { callStore } from '../../stores/call';
@@ -126,7 +132,7 @@ const CallScreen: React.FC = () => {
       <View style={styles.background} />
 
       {/* Remote video - full screen */}
-      {showRemoteVideo && remoteVideoTrack && (
+      {showRemoteVideo && remoteVideoTrack && VideoView && (
         <VideoView
           videoTrack={remoteVideoTrack}
           style={styles.fullScreenVideo}
@@ -135,7 +141,7 @@ const CallScreen: React.FC = () => {
       )}
 
       {/* Local video - picture in picture */}
-      {showLocalVideo && localVideoTrack && (
+      {showLocalVideo && localVideoTrack && VideoView && (
         <View style={styles.localVideoContainer}>
           <VideoView
             videoTrack={localVideoTrack}