Let sidebar and queue show the same ambient backdrop as the center

Sidebar and Queue painted their own opaque bg-bg, which sat in front of
the fixed, full-viewport app-backdrop div (z-index: -10) that carries
--mf-ambient's radial-gradient glow in dark mode. Since the main content
area has no background of its own, the glow only ever showed through
there — sidebar and queue blocked it with a flat, gradient-less fill of
the same base color.

Drop bg-bg from both so the same app-backdrop (color + ambient gradient)
shows through underneath them too. In light mode this is a no-op
(--mf-ambient is 'none' and app-backdrop's flat color already matches
what bg-bg painted), so only dark mode is visually affected.

web-player/src/lib/components/layout/Sidebar.svelte

@@ -31,7 +31,7 @@
 </script>
 
 <aside
-	class="hidden shrink-0 flex-col border-r border-hairline bg-bg p-5 lg:flex"
+	class="hidden shrink-0 flex-col border-r border-hairline p-5 lg:flex"
 	style="width:var(--mf-sidebar-w)"
 >
 	<nav class="flex flex-col gap-1">