fix: crossfade now-playing background instead of hard flash

Two-layer opacity crossfade replaces the broken background-image transition.
New image loads on the hidden layer, then both opacity values swap smoothly.

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

src/player.js

@@ -57,17 +57,28 @@ export function updatePlayerUI(s) {
   _syncNpDirect(s);
 }
 
+let _npBgCurr = 'a';
+function _crossfadeNpBg(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('${cv}')` : 'none';
+  nextEl.style.opacity = '1';
+  currEl.style.opacity = '0';
+  _npBgCurr = nextId;
+}
+
 export function _syncNpDirect(s) {
   if(!S.npOpen || !s) return;
   const cv = coverUrl(s.coverArt, 400);
   const cover = document.getElementById('np-cover');
-  const bg    = document.getElementById('np-bg');
   if (cover) {
     const existingImg = cover.querySelector('img');
     if (cv && existingImg) existingImg.src = cv;
     else if (cover) cover.innerHTML = cv ? `<img src="${cv}" style="width:100%;height:100%;object-fit:cover">` : '<div class="no-art">♪</div>';
   }
-  if(bg) bg.style.backgroundImage = cv ? `url('${cv}')` : 'none';
+  _crossfadeNpBg(cv);
   const t = document.getElementById('np-title'); if(t) t.textContent = s.title || '—';
   const an = document.getElementById('np-artist-name');
   if(an) {