refactor: single-track ExoPlayer model — JS owns queue entirely

Root cause of all Android player bugs: two competing queue managers.
ExoPlayer held all tracks and auto-advanced with its own index while JS
tracked S.qIdx separately. seekToIndex() is async via runOnMain, so
skips would interleave with ExoPlayer's own advances — causing desync,
wrong-song display, and skips that silently did nothing.

Fix: ExoPlayer always holds exactly ONE MediaItem. JS is the single
source of truth for queue order, shuffle, and repeat.

JS changes (player.js):
- playQueue/playCurrent/nextSong/prevSong/toggleShuffle: replace
  setQueue(allTracks, idx) + seekToIndex with setQueue([single], 0)
- _pushQueueIfBridge: send only current track
- nativeInsertAt: no-op (radio songs land in S.queue; JS picks them up)
- fillRadioQueue: remove addToQueue native call
- onTrackChanged: simplified to just schedule scrobble + radio top-up
- onPlaybackStateChanged: remove index-reconciliation block
- onPositionUpdate: remove reconciliation tick (no queue drift possible)
- onQueueEnded: call onEnded() — JS drives what plays next
- toggleRepeat: setRepeatMode('one') only for repeat=one, else 'off'
- Add onNotificationNext / onNotificationPrev to NSPlayer
- Remove _skipping/_skipTimer/_reconTick/_setSkipping (all dead code)

Android changes:
- MusicService: add onSkipToNext/onSkipToPrevious to StateListener;
  add MediaSession.Callback to intercept notification skip buttons and
  delegate them to JS instead of letting ExoPlayer handle them
- MainActivity: implement new StateListener methods → callJs()

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

android/app/src/main/kotlin/com/novasonic/app/MainActivity.kt

@@ -215,6 +215,9 @@ class MainActivity : AppCompatActivity(), MusicService.StateListener {
 
     override fun onError(message: String) = callJs("onError", message)
 
+    override fun onSkipToNext()     = callJs("onNotificationNext")
+    override fun onSkipToPrevious() = callJs("onNotificationPrev")
+
     // ─── JS bridge helpers ────────────────────────────────────────────────────
 
     /**

android/app/src/main/kotlin/com/novasonic/app/MusicService.kt

@@ -19,6 +19,7 @@ import androidx.media3.session.CacheBitmapLoader
 import androidx.media3.session.MediaSession
 import androidx.media3.session.MediaSessionService
 import com.google.common.util.concurrent.MoreExecutors
+import com.google.common.util.concurrent.Futures
 import java.util.concurrent.Executors
 
 class MusicService : MediaSessionService() {
@@ -32,6 +33,8 @@ class MusicService : MediaSessionService() {
         fun onPositionUpdate(positionMs: Long, durationMs: Long)
         fun onQueueEnded()
         fun onError(message: String)
+        fun onSkipToNext()
+        fun onSkipToPrevious()
     }
 
     companion object {
@@ -106,8 +109,22 @@ class MusicService : MediaSessionService() {
         )
 
         mediaSession = MediaSession.Builder(this, player)
-            .setBitmapLoader(bitmapLoader)        // use cached loader for notification artwork
+            .setBitmapLoader(bitmapLoader)
             .setSessionActivity(notifTapPending)
+            .setCallback(object : MediaSession.Callback {
+                // Intercept notification next/prev so JS (not ExoPlayer) handles queue logic.
+                override fun onSkipToNextMediaItem(
+                    session: MediaSession, controller: MediaSession.ControllerInfo
+                ) = Futures.immediateFuture(androidx.media3.session.SessionResult(
+                    androidx.media3.session.SessionResult.RESULT_SUCCESS
+                )).also { stateListener?.onSkipToNext() }
+
+                override fun onSkipToPreviousMediaItem(
+                    session: MediaSession, controller: MediaSession.ControllerInfo
+                ) = Futures.immediateFuture(androidx.media3.session.SessionResult(
+                    androidx.media3.session.SessionResult.RESULT_SUCCESS
+                )).also { stateListener?.onSkipToPrevious() }
+            })
             .build()
 
         player.addListener(PlayerListener())