fix: prevent NP overlay from flickering through to content below on Android WebView

Replace will-change:transform with translate3d(0,100%,0) / translate3d(0,0,0).
will-change triggers a layer-creation/destruction cycle on WebView — when the
player bar DOM updates during a skip, the compositor can briefly expose the
layer's clear back buffer, flashing the content below through the overlay.
translate3d pins the element to a stable GPU layer from the start.
Also add contain:layout style paint to isolate the overlay's paint context
from external repaints, and -webkit-backface-visibility for older WebView.

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

styles.css

@@ -573,14 +573,16 @@ button:focus{outline:none}
   position:fixed;inset:0;z-index:300;
   display:flex;flex-direction:column;
   background:var(--bg);
-  transform:translateY(100%);
+  /* translate3d keeps the element on a stable GPU layer without will-change's
+     layer-creation/destruction cycle that causes WebView to flash the content below */
+  transform:translate3d(0,100%,0);
   transition:transform .35s cubic-bezier(.4,0,.2,1);
   overflow:hidden;
-  will-change:transform;
-  backface-visibility:hidden;
+  backface-visibility:hidden;-webkit-backface-visibility:hidden;
+  contain:layout style paint;
   isolation:isolate;
 }
-.np-overlay.open{transform:translateY(0)}
+.np-overlay.open{transform:translate3d(0,0,0)}
 
 /* Blurred album art as background — two layers for crossfade */
 .np-bg{