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/package.json

@@ -19,18 +19,15 @@
     "react-native-gesture-handler": "~2.31.1",
     "react-native-safe-area-context": "~5.7.0",
     "react-native-screens": "^4.25.2",
-    "react-native-track-player": "^4.1.2",
+    "react-native-track-player": "^5.0.0-alpha0",
     "zustand": "^5.0.14"
   },
   "scripts": {
     "start": "expo start",
     "android": "expo start --android",
     "ios": "expo start --ios",
-    "web": "expo start --web",
-    "postinstall": "patch-package"
+    "web": "expo start --web"
   },
   "private": true,
-  "devDependencies": {
-    "patch-package": "^8.0.1"
-  }
+  "devDependencies": {}
 }