fix: eliminate mobile bg flash — double rAF + will-change:opacity

new Image() onload fires when bytes are downloaded but GPU decode of the
blurred layer hasn't happened yet on mobile. Double rAF defers the opacity
swap until after the browser has actually painted the new background.
will-change:opacity keeps the layers on dedicated compositor layers so
the blur doesn't need to be recomputed on every opacity transition.

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

src/player.js

@@ -68,9 +68,12 @@ function _crossfadeNpBg(cv) {
     const nextEl = document.getElementById(`np-bg-${nextId}`);
     if (!currEl || !nextEl) return;
     nextEl.style.backgroundImage = cv ? `url('${cv}')` : 'none';
-    nextEl.style.opacity = '1';
-    currEl.style.opacity = '0';
-    _npBgCurr = nextId;
+    // Double rAF: wait for browser to paint the new bg (GPU decode) before fading
+    requestAnimationFrame(() => requestAnimationFrame(() => {
+      nextEl.style.opacity = '1';
+      currEl.style.opacity = '0';
+      _npBgCurr = nextId;
+    }));
   };
   if (!cv) { doFade(); return; }
   const img = new Image();

styles.css

@@ -515,6 +515,7 @@ nav{flex:1;overflow-y:auto;padding:4px 10px}
   filter:blur(60px) brightness(.35) saturate(1.4);
   transform:scale(1.1);
   transition:opacity .55s ease;
+  will-change:opacity;
   z-index:0;
 }