feat: Implement native Android MediaSession and foreground service for TTS playback

- Add `ReaderTtsMediaService` to handle background playback, media controls, and notifications on Android
- Integrate `MediaSessionCompat` to support external media controls and lock screen integration
- Add `ReaderTtsMediaBridge` for synchronized state communication between Kotlin and Flutter
- Update `TtsNotifier` to use the native Android service when available, with a fallback for other platforms
- Implement sentence-level highlighting and tapping to start reading from a specific location
- Update Android manifest with necessary permissions for foreground services and notifications
- Adjust TTS speech rate constants and improve playback health monitoring and recovery logic

android/app/src/main/AndroidManifest.xml

@@ -2,6 +2,9 @@
     <uses-permission android:name="android.permission.INTERNET"/>
     <uses-permission android:name="android.permission.WAKE_LOCK"/>
     <uses-permission android:name="android.permission.REQUEST_IGNORE_BATTERY_OPTIMIZATIONS"/>
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE"/>
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK"/>
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS"/>
     <application
         android:label="reader_app"
         android:name="${applicationName}"
@@ -34,6 +37,10 @@
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />
+        <service
+            android:name=".tts.ReaderTtsMediaService"
+            android:exported="false"
+            android:foregroundServiceType="mediaPlayback" />
     </application>
     <!-- Required to query activities that can process text, see:
          https://developer.android.com/training/package-visibility and