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>

styles.css

@@ -508,13 +508,13 @@ nav{flex:1;overflow-y:auto;padding:4px 10px}
 }
 .np-overlay.open{transform:translateY(0)}
 
-/* Blurred album art as background */
+/* Blurred album art as background — two layers for crossfade */
 .np-bg{
   position:absolute;inset:-40px;
   background-size:cover;background-position:center;
   filter:blur(60px) brightness(.35) saturate(1.4);
   transform:scale(1.1);
-  transition:background-image .6s ease;
+  transition:opacity .55s ease;
   z-index:0;
 }