fix: preload bg image before crossfade to eliminate black flash

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

src/player.js

@@ -62,6 +62,7 @@ let _npBgLastUrl = '';
 function _crossfadeNpBg(cv) {
   if ((cv || '') === _npBgLastUrl) return;
   _npBgLastUrl = cv || '';
+  const doFade = () => {
     const currEl = document.getElementById(`np-bg-${_npBgCurr}`);
     const nextId = _npBgCurr === 'a' ? 'b' : 'a';
     const nextEl = document.getElementById(`np-bg-${nextId}`);
@@ -70,6 +71,12 @@ function _crossfadeNpBg(cv) {
     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) {