fix(player-bar): contain paint to prevent black-flash on WebKitGTK

The player bar occasionally renders fully black for one frame on Linux
(WebKitGTK with software compositing) when an unrelated layer elsewhere
in the page invalidates. `contain: layout paint` makes the bar its own
paint boundary so it can no longer be pulled into a surrounding dirty
rect.

No-op on Wayland-with-GPU and on Chromium-based webviews (Windows,
macOS) — those don't exhibit the flash to begin with.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/styles/layout.css

@@ -1145,6 +1145,11 @@
   height: var(--player-height);
   position: relative;
   z-index: 100;
+  /* WebKitGTK (software compositing) occasionally paints the bar fully
+     black for one frame when an unrelated layer in the page invalidates.
+     `contain` isolates layout/paint so the bar's region cannot
+     participate in the surrounding dirty rect. */
+  contain: layout paint;
 }
 
 .player-track-info {