feat: Expo mobile scaffold — Phase 2 complete (118-file structure)

Expo SDK 51 TypeScript scaffold with all architecture files.
Verified: tsc 0 errors, jest passes.

mobile/babel.config.js

@@ -0,0 +1,9 @@
+module.exports = function (api) {
+  api.cache(true);
+  return {
+    presets: ['babel-preset-expo'],
+    plugins: [
+      'react-native-reanimated/plugin'
+    ]
+  };
+};