fix: preload bg image before crossfade to eliminate black flash

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

src/player.js

@@ -62,14 +62,21 @@ let _npBgLastUrl = '';
 function _crossfadeNpBg(cv) {
   if ((cv || '') === _npBgLastUrl) return;
   _npBgLastUrl = cv || '';
-  const currEl = document.getElementById(`np-bg-${_npBgCurr}`);
-  const nextId = _npBgCurr === 'a' ? 'b' : 'a';
-  const nextEl = document.getElementById(`np-bg-${nextId}`);
-  if (!currEl || !nextEl) return;
-  nextEl.style.backgroundImage = cv ? `url('${cv}')` : 'none';
-  nextEl.style.opacity = '1';
-  currEl.style.opacity = '0';
-  _npBgCurr = nextId;
+  const doFade = () => {
+    const currEl = document.getElementById(`np-bg-${_npBgCurr}`);
+    const nextId = _npBgCurr === 'a' ? 'b' : 'a';
+    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;
+  };
+  if (!cv) { doFade(); return; }
+  const img = new Image();
+  img.onload = doFade;
+  img.onerror = doFade;
+  img.src = cv;
 }
 
 export function _syncNpDirect(s) {