feat: native Android audio via Media3 ExoPlayer + full NSBridge JS adapter

- Replace legacy MediaBrowserServiceCompat/MediaSessionCompat with
  Media3 MediaSessionService (MusicService.kt) + ExoPlayer set to
  WAKE_MODE_NETWORK — holds WakeLock + WifiLock through Doze so audio
  plays uninterrupted on locked screen
- Media3 manages the notification; no manual NotificationCompat.Builder
  → no disappear/reappear flicker when skipping tracks
- Full bidirectional bridge:
    JS → native: window.NSBridge.* (WebBridge.kt, 13 methods)
    Native → JS: window.NSPlayer.* callbacks (onReady, onTrackChanged,
      onPositionUpdate, onPlaybackStateChanged, onQueueEnded, onError)
- player.js: progressive enhancement — all playback routes through
  NSBridge when running inside the wrapper, falls back to S.audio on
  plain PWA/browser; removes all window.Android.* calls
- main.js: guard _onForeground mediaSession playbackState setter behind
  !window.NSBridge so native Media3 session is never stomped
- ProGuard rules updated to keep WebBridge @JavascriptInterface methods
  and Media3 classes
- bridge-test.html: manual integration test page for every bridge method

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

android/app/proguard-rules.pro

@@ -0,0 +1,8 @@
+# Keep @JavascriptInterface methods — called by WebView via reflection, stripped otherwise.
+-keepclassmembers class com.novasonic.app.WebBridge {
+    @android.webkit.JavascriptInterface <methods>;
+}
+
+# Keep Media3 session / exoplayer classes used by reflection / service binding.
+-keep class androidx.media3.** { *; }
+-keep class com.google.android.exoplayer2.** { *; }