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.

app/_layout.tsx

@@ -1,3 +1,4 @@
+import '../src/polyfills';
 import React, { useEffect, useRef, useState } from 'react';
 import { AppState, AppStateStatus, Platform, View, ActivityIndicator } from 'react-native';
 import { Stack, useRouter } from 'expo-router';