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/player/Queue.svelte

@@ -18,7 +18,7 @@
 {#if queuePanel.open}
 	<aside
 		transition:fly={{ x: 24, duration: 220, easing: cubicOut }}
-		class="hidden shrink-0 flex-col border-l border-hairline bg-bg px-4 pt-5 sm:flex"
+		class="hidden shrink-0 flex-col border-l border-hairline px-4 pt-5 sm:flex"
 		style="width:var(--mf-queue-w); padding-bottom:calc(var(--mf-player-h) + 2rem)"
 	>
 		<div class="mb-5 flex items-center justify-between">