fix: upgrade to react-native-track-player 5.0.0-alpha0 (Media3/New Arch)

- RNTP 4.1.2 used ExoPlayer 2.x which is incompatible with RN 0.85.3
- RNTP 5.0.0-alpha0 uses androidx.media3 and supports New Architecture
- Remove newArchEnabled:false from app.json (New Arch is default)
- Remove Kotlin null safety patch (no longer needed)
- Remove patch-package (no more patches required)
- Fix getState() -> getPlaybackState() API rename in player.js
- Add babel.config.js with babel-preset-expo

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mobile/babel.config.js

@@ -0,0 +1,6 @@
+module.exports = function (api) {
+  api.cache(true);
+  return {
+    presets: ['babel-preset-expo'],
+  };
+};