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/babel.config.js

@@ -2,9 +2,5 @@ module.exports = function (api) {
   api.cache(true);
   return {
     presets: ['babel-preset-expo'],
-    plugins: [
-      // expo-router requires this plugin
-      'expo-router/babel',
-    ],
   };
 };