fix: deduplicate NP background crossfade — skip if URL unchanged

_syncNpDirect is called 2-3x per skip from different call sites;
the crossfade now no-ops when the cover URL hasn't changed.

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

src/player.js

@@ -58,7 +58,10 @@ export function updatePlayerUI(s) {
 }
 
 let _npBgCurr = 'a';
+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}`);