fix: Samsung notification stays alive; retry play() on background rejection

- Set playbackState='playing' explicitly before MediaMetadata swap so
  Samsung One UI never sees a gap where state is ambiguous during skip
- Add retry loop (500ms→1s→2s→4s→8s) in playCurrent() catch — handles
  Android Doze rejecting play() in background; stops retrying if the
  song changes mid-flight (AbortError)
- Fix error handler: set _trackChanging=true before audio.load() so
  the load()'s pause event can't slip past the Samsung notification guard

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

src/player.js

@@ -52,6 +52,8 @@ export function playCurrent() {
   if(S.qIdx < 0 || S.qIdx >= S.queue.length) return;
   const s = S.queue[S.qIdx];
   S._trackChanging = true;
+  // Explicitly keep Samsung notification alive — set playing BEFORE metadata swap
+  if(navigator.mediaSession) navigator.mediaSession.playbackState = 'playing';
   updatePlayerUI(s);        // update metadata/notification BEFORE src change
   onNpSongChange(s.id);
   renderQueue();
@@ -59,7 +61,20 @@ export function playCurrent() {
   const src = (_preId === s.id && _preUrl) ? _preUrl : streamUrl(s.id);
   _clearPreload();
   S.audio.src = src;
-  S.audio.play().catch(() => { S._trackChanging = false; });
+  const _tryPlay = (attempt) => {
+    S.audio.play().catch(err => {
+      S._trackChanging = false;
+      if (err?.name === 'AbortError') return; // src changed mid-flight, stop
+      if (attempt < 4 && S.queue[S.qIdx]?.id === s.id) {
+        setTimeout(() => {
+          if (S.queue[S.qIdx]?.id !== s.id) return;
+          S._trackChanging = true;
+          _tryPlay(attempt + 1);
+        }, Math.min(500 * Math.pow(2, attempt), 8000)); // 500ms, 1s, 2s, 4s, 8s
+      }
+    });
+  };
+  _tryPlay(0);
 }
 
 export function updatePlayerUI(s) {