fix(mobile): add @expo/metro-runtime, drop deprecated expo-router/babel

expo-router@4 imports @expo/metro-runtime as the very first thing in its
entry-classic.js so Fast Refresh works; bundler errored without it.
Also dropped expo-router/babel from babel.config.js — its functionality
moved into babel-preset-expo in SDK 50.

apps/mobile/package.json

@@ -21,6 +21,7 @@
     "@chat-app/db-types": "workspace:*",
     "@chat-app/shared": "workspace:*",
     "@config-plugins/react-native-webrtc": "^10.0.0",
+    "@expo/metro-runtime": "^55.0.11",
     "@livekit/react-native": "^2.10.3",
     "@livekit/react-native-webrtc": "^144.0.0",
     "@react-native-async-storage/async-storage": "^1.23.1",