fix: eliminate NP overlay stale-song on autoplay and cover-load delay

Three improvements:
- openNowPlaying(): reads NSBridge.getState() (always-fresh @Volatile snapshot)
  via ID search to reconcile S.qIdx before render; fixes NP showing old song
  when deferred evaluateJavascript callbacks haven't fired yet after backgrounding
- onPositionUpdate(): periodic check (~every 2s) detects track mismatch via
  snapshot ID; catches missed/delayed onTrackChanged when NP is open
- updatePlayerUI(): pre-warms next song's 1024px cover so auto-advance NP
  swap is instant from browser cache instead of a 1-second load delay
- styles: add backface-visibility:hidden + isolation:isolate to .np-overlay
  to reduce GPU composite flicker on Android WebView on manual skip

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

src/player.js

@@ -16,6 +16,7 @@ let _lastPositionMs = 0;   // ms — set by onPositionUpdate, used for prev() de
 let _lastDurationMs = 0;   // ms — set by onPositionUpdate, used for seek calculations
 let _skipping       = false;  // true briefly after a skip — suppresses the transient isPlaying:false
 let _skipTimer      = null;
+let _reconTick      = 0;   // counter for periodic track-reconciliation inside onPositionUpdate
 
 // ─── SCROBBLE ─────────────────────────────────────────────────────────────
 let _scrobbleTimer  = null;
@@ -221,6 +222,24 @@ window.NSPlayer = {
     _lastPositionMs = posMs;
     _lastDurationMs = durMs;
     if (S._scrubbing) return;
+    // Every 8 ticks (~2 s): verify JS track matches native snapshot.
+    // Catches the gap between auto-advance and onTrackChanged arriving in JS
+    // (evaluateJavascript is deferred while WebView is backgrounded/resuming).
+    if (HAS_BRIDGE && ++_reconTick >= 8) {
+      _reconTick = 0;
+      try {
+        const snap   = JSON.parse(window.NSBridge.getState() || '{}');
+        const snapId = String(snap.currentId || '');
+        if (snapId && snapId !== String(S.queue[S.qIdx]?.id || '')) {
+          const found = S.queue.findIndex(s => String(s.id) === snapId);
+          if (found >= 0) {
+            S.qIdx = found;
+            const s = S.queue[S.qIdx];
+            if (s) { updatePlayerUI(s, true); syncNowPlaying(); onNpSongChange(s.id); renderQueue(); }
+          }
+        }
+      } catch {}
+    }
     const dur = durMs / 1000;
     const cur = posMs / 1000;
     if (dur <= 0) return;
@@ -496,6 +515,12 @@ export function updatePlayerUI(s, _skipMediaSession = false) {
   // Preload the hi-res NP cover now so opening the overlay is instant
   const _npCv = coverUrl(s.coverArt, 1024);
   if (_npCv && _npCv !== _npBgLastUrl) { const _pre = new Image(); _pre.src = _npCv; }
+  // Pre-warm the next song's 1024 cover so auto-advance NP swap is instant from cache
+  const _nextForPreload = S.qIdx + 1 < S.queue.length ? S.queue[S.qIdx + 1] : null;
+  if (_nextForPreload?.coverArt) {
+    const _nextCv = coverUrl(_nextForPreload.coverArt, 1024);
+    if (_nextCv) { const _preN = new Image(); _preN.src = _nextCv; }
+  }
 }
 
 // ── Solid cover-color background ────────────────────────────────────────────