perf(web): instant skips via gapless handoff to a pre-buffered element

Reintroduce the two-element handoff, this time as a skip-speed optimization
(not for the OS tile). A second <audio> element pre-buffers the next track;
on a forward skip playCurrent hands playback over to it instead of reloading
the source, so playback starts immediately instead of ~1s. Covers are
pre-warmed in parallel so the new artwork is ready too.

Listeners are bound to both elements once (handlers guard with el !== S.audio).
Safety: handoff only when the standby is buffered (readyState >= 2); otherwise
falls back to the unchanged source-swap path, and a failed handoff play()
recovers with a fresh source load. Web-only; the native bridge path is untouched.

Note: this does not change the OS media-tile flicker (a separate, MSE-only fix).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.js

@@ -122,6 +122,10 @@ window.addEventListener('load', () => {
 
   // Wire up audio elements from DOM
   S.audio        = document.getElementById('audio-element');
+  const _audioB  = document.getElementById('audio-element-b');
+  // Two fixed audio elements for instant skips — S.audio always points at the active one,
+  // the other pre-buffers the next track so a skip can hand off to it without reloading.
+  S._audioEls    = _audioB ? [S.audio, _audioB] : [S.audio];
   S.previewAudio = document.getElementById('preview-audio');
 
   const saved = localStorage.getItem('ns_cfg');
@@ -134,44 +138,51 @@ window.addEventListener('load', () => {
   if(window.NS_SERVER_URL?.trim()) { const el=document.getElementById('srv-url'); if(el&&!el.value) el.value=window.NS_SERVER_URL.trim(); }
 
   document.getElementById('srv-pass').addEventListener('keydown', e => { if(e.key==='Enter') doConnect(); });
-  S.audio.addEventListener('timeupdate', updateProgress);
-  S.audio.addEventListener('ended', onEnded);
-  S.audio.addEventListener('play', () => {
-    S._trackChanging = false;
-    absInterrupt();
-    if (S.previewCurrent) stopPreview(false);
-    setPlayIcon(true);
-    document.getElementById('player-art').classList.add('spinning');
-    if(navigator.mediaSession) navigator.mediaSession.playbackState = 'playing';
-  });
-  S.audio.addEventListener('pause', () => {
-    setPlayIcon(false);
-    document.getElementById('player-art').classList.remove('spinning');
-    // Suppress during track change — src swap fires pause before the new track starts
-    if(!S._trackChanging && navigator.mediaSession) navigator.mediaSession.playbackState = 'paused';
-  });
+  // Listeners are bound to BOTH audio elements so a skip can hand off to the pre-buffered
+  // one with no rebinding. Handlers that only apply to the playing element guard with
+  // `el !== S.audio` (the idle standby element must not affect UI/recovery).
+  let _errRetries = 0, _errTimer = null;
+  function bindAudioEl(el) {
+    el.addEventListener('timeupdate', updateProgress);
+    el.addEventListener('ended', onEnded);
+    el.addEventListener('play', () => {
+      S._trackChanging = false;
+      absInterrupt();
+      if (S.previewCurrent) stopPreview(false);
+      setPlayIcon(true);
+      document.getElementById('player-art').classList.add('spinning');
+      if(navigator.mediaSession) navigator.mediaSession.playbackState = 'playing';
+    });
+    el.addEventListener('pause', () => {
+      if (el !== S.audio) return;   // ignore the idle standby element during a handoff
+      setPlayIcon(false);
+      document.getElementById('player-art').classList.remove('spinning');
+      // Suppress during track change — src swap fires pause before the new track starts
+      if(!S._trackChanging && navigator.mediaSession) navigator.mediaSession.playbackState = 'paused';
+    });
+    // stalled = browser buffer ran out briefly; let the browser retry naturally.
+    el.addEventListener('play', () => { _errRetries = 0; clearTimeout(_errTimer); }, { capture: true });
+    el.addEventListener('error', () => {
+      if (el !== S.audio) return;   // standby/preload errors must not disrupt active playback
+      // Skip if playCurrent() is already handling recovery (_trackChanging = true means
+      // we're mid-transition or waiting for canplay — don't conflict with that)
+      if(!S.queue[S.qIdx] || S._trackChanging) return;
+      _errRetries++;
+      const delay = Math.min(1000 * Math.pow(2, _errRetries - 1), 8000); // 1s, 2s, 4s, 8s
+      clearTimeout(_errTimer);
+      _errTimer = setTimeout(() => {
+        if(!S.queue[S.qIdx] || S._trackChanging) return;
+        S._trackChanging = true;
+        S.audio.load();
+        S.audio.play().catch(() => { S._trackChanging = false; });
+      }, delay);
+    });
+  }
+  S._audioEls.forEach(bindAudioEl);
+
   S.previewAudio.addEventListener('timeupdate', updatePreviewProgress);
   S.previewAudio.addEventListener('ended', stopPreview);
 
-  // stalled = browser buffer ran out briefly; let the browser retry naturally.
-  // The watchdog below catches genuine long freezes.
-  let _errRetries = 0, _errTimer = null;
-  S.audio.addEventListener('play', () => { _errRetries = 0; clearTimeout(_errTimer); }, { capture: true });
-  S.audio.addEventListener('error', () => {
-    // Skip if playCurrent() is already handling recovery (_trackChanging = true means
-    // we're mid-transition or waiting for canplay — don't conflict with that)
-    if(!S.queue[S.qIdx] || S._trackChanging) return;
-    _errRetries++;
-    const delay = Math.min(1000 * Math.pow(2, _errRetries - 1), 8000); // 1s, 2s, 4s, 8s
-    clearTimeout(_errTimer);
-    _errTimer = setTimeout(() => {
-      if(!S.queue[S.qIdx] || S._trackChanging) return;
-      S._trackChanging = true;
-      S.audio.load();
-      S.audio.play().catch(() => { S._trackChanging = false; });
-    }, delay);
-  });
-
   const _savedVol = parseInt(localStorage.getItem('ns_volume') ?? '', 10);
   setVolume(isNaN(_savedVol) ? 55 : _savedVol);