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>

index.html

@@ -613,7 +613,8 @@
 
 <!-- NOW PLAYING OVERLAY -->
 <div class="np-overlay" id="np-overlay">
-  <div class="np-bg" id="np-bg"></div>
+  <div class="np-bg" id="np-bg-a"></div>
+  <div class="np-bg" id="np-bg-b" style="opacity:0"></div>
   <div class="np-inner">
     <div class="np-topbar">
       <button class="np-close" onclick="closeNowPlaying()" title="Schließen">