fix: lock screen audio + notification — audioSession once, deferred metadata, SW artwork cache

- Set navigator.audioSession.type='playback' only once (guarded flag) — repeated
  reassignment was resetting the OS audio session and triggering Doze network cuts
- Defer updateMediaSession() until after play() resolves, not before audio.src swap —
  new MediaMetadata during HAVE_NOTHING state caused Samsung to close/reopen notification
- Fix silent bug: updatePlayerUI(s, true) now actually skips MediaSession (added param)
- 10s safety guards now attempt play() instead of just resetting _trackChanging flag
- SW v40: cache-first for getCoverArt with token-stripped keys — artwork served from
  cache even when Doze blocks network, survives session restarts with new auth tokens
- Pre-warm next track artwork 2s after song starts (earlier than 30s-remaining trigger)

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

src/main.js

@@ -127,8 +127,6 @@ window.addEventListener('load', () => {
   S.audio.addEventListener('ended', onEnded);
   S.audio.addEventListener('play', () => {
     S._trackChanging = false;
-    // Tell OS this is a music playback app — keeps background network alive (Doze mode)
-    try { if(navigator.audioSession) navigator.audioSession.type = 'playback'; } catch {}
     absInterrupt();
     setPlayIcon(true);
     document.getElementById('player-art').classList.add('spinning');

src/player.js

@@ -16,6 +16,16 @@ function _cachedCoverUrl(id, size) {
   return _coverCache.get(key);
 }
 
+// ─── AUDIO SESSION — set once, never again ───
+// Reassigning audioSession.type on every track change resets the OS audio session,
+// which can cut background network briefly — exactly what causes Doze to kill autoplay.
+let _audioSessionSet = false;
+function _setAudioSession() {
+  if (_audioSessionSet) return;
+  _audioSessionSet = true;
+  try { if(navigator.audioSession) navigator.audioSession.type = 'playback'; } catch {}
+}
+
 // ─── PRELOAD (resource hint keeps connection open through Doze gap) ───
 let _preUrl  = '';
 let _preId   = '';
@@ -74,37 +84,65 @@ export function playCurrent() {
   const s = S.queue[S.qIdx];
   const gen = ++_playGen; // capture generation — any callback that sees gen !== _playGen is stale
   S._trackChanging = true;
-  // Keep Samsung notification alive — set playing BEFORE metadata swap
+  // Keep Samsung notification alive — set playing BEFORE any metadata swap
   if(navigator.mediaSession) navigator.mediaSession.playbackState = 'playing';
-  updatePlayerUI(s);
+  // Update player bar DOM but NOT MediaSession metadata yet — setting metadata while the
+  // audio element is about to enter HAVE_NOTHING (src change) causes Chrome to briefly
+  // signal "stopped" internally, which makes Samsung close and re-open the lock screen
+  // notification. We defer updateMediaSession() until after play() actually resolves.
+  updatePlayerUI(s, true);
   onNpSongChange(s.id);
   renderQueue();
   const src = (_preId === s.id && _preUrl) ? _preUrl : streamUrl(s.id);
   _clearPreload();
   S.audio.src = src;
 
-  // Only resets _trackChanging if no newer playCurrent() has started since.
-  // Prevents rapid skips from letting a stale reset() clobber the active transition.
-  const _reset = () => { if (_playGen === gen) S._trackChanging = false; };
+  const _resetFlag = () => { if (_playGen === gen) S._trackChanging = false; };
 
-  // Wait for canplay (audio engine — not throttled by Android background timers).
-  // _trackChanging stays TRUE so Android audio-focus-loss pause events can't reach
-  // Samsung and dismiss the lock screen notification during recovery.
+  // Called once audio is confirmed playing — safe moment to swap MediaSession metadata
+  // and kick off next-track preload (artwork already warm via the 2s timer below).
+  const _applySession = () => {
+    if (_playGen !== gen) return;
+    updateMediaSession(s);
+  };
+
+  // Wait for canplay (audio engine event — not throttled by Android background timers).
+  // _trackChanging stays TRUE throughout so spurious pause events during buffering can't
+  // reach Samsung and dismiss the lock screen notification.
   function _waitCanPlay() {
-    const guard = setTimeout(_reset, 10000); // 10s Kima-style failsafe
+    const guard = setTimeout(() => {
+      // canplay didn't fire within 10s (Doze likely stalled the network).
+      // Last-ditch: try play() anyway — the element may have buffered enough already.
+      // If it fails, _onForeground() will retry when the user opens the app.
+      if (_playGen !== gen) return;
+      S._trackChanging = false;
+      S.audio.play().then(_applySession).catch(() => {});
+    }, 10000);
     S.audio.addEventListener('canplay', () => {
       clearTimeout(guard);
       if (_playGen !== gen) return; // newer playCurrent() took over — do nothing
-      if (!S.audio.paused) return;  // already playing — play event will clear flag
-      try { if(navigator.audioSession) navigator.audioSession.type = 'playback'; } catch {}
-      S.audio.play().catch(_reset);
+      if (!S.audio.paused) { _applySession(); return; } // already playing
+      _setAudioSession();
+      S.audio.play().then(_applySession).catch(_resetFlag);
     }, { once: true });
   }
 
-  const _initGuard = setTimeout(_reset, 10000);
-  try { if(navigator.audioSession) navigator.audioSession.type = 'playback'; } catch {}
+  const _initGuard = setTimeout(() => {
+    // play() neither resolved nor rejected in 10s — hardware stall, try once more
+    if (_playGen !== gen) return;
+    S._trackChanging = false;
+    S.audio.play().then(_applySession).catch(() => {});
+  }, 10000);
+
+  _setAudioSession();
   S.audio.play()
-    .then(() => clearTimeout(_initGuard))
+    .then(() => {
+      clearTimeout(_initGuard);
+      _applySession();
+      // Pre-warm next track artwork 2s into playback — early enough for lock screen
+      // notification to find it in cache on the next skip
+      setTimeout(_preloadNext, 2000);
+    })
     .catch(err => {
       clearTimeout(_initGuard);
       if (_playGen !== gen) return; // superseded — let the newer call handle everything
@@ -118,7 +156,7 @@ export function playCurrent() {
     });
 }
 
-export function updatePlayerUI(s) {
+export function updatePlayerUI(s, _skipMediaSession = false) {
   document.getElementById('player-title').textContent = s.title;
   const artistParts = (s.artist || '').split(/\s*(?:\/|•|,)\s*/).filter(Boolean);
   document.getElementById('player-artist-name').innerHTML = artistParts.length
@@ -136,7 +174,7 @@ export function updatePlayerUI(s) {
   else art.innerHTML = cv ? `<img src="${cv}" style="width:100%;height:100%;object-fit:cover">` : '<div class="no-art">♪</div>';
   updateStarBtn();
   document.title = `${s.title} — NovaSonic`;
-  updateMediaSession(s);
+  if (!_skipMediaSession) updateMediaSession(s);
   if(document.getElementById('page-recommendations').classList.contains('active')) loadRecommendations();
   try {
     localStorage.setItem('ns_last_played', JSON.stringify({