Revert chrome-bg: sidebar/queue should just be the tinted background

Per feedback, the solid color-mix bar was wrong — sidebar, queue and
the mobile header should stay on the plain --mf-bg (the same subtly
hue-shifting black used everywhere else), exactly like light mode
already does, not a separately-boosted colored panel. Drop --mf-chrome-bg/
--color-chrome and switch those three back to bg-bg.

The general dark-mode richness bump (tc(), --mf-bg/--mf-elevated/etc.)
and the glass-bar background-color fix from the previous commit stay;
only the separate chrome treatment is reverted.

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

@@ -18,7 +18,7 @@
 </script>
 
 <header
-	class="sticky top-0 z-(--z-sticky) flex h-14 items-center justify-between border-b border-hairline bg-chrome/85 page-x backdrop-blur-md sm:hidden"
+	class="sticky top-0 z-(--z-sticky) flex h-14 items-center justify-between border-b border-hairline bg-bg/85 page-x backdrop-blur-md sm:hidden"
 >
 	<a href="/">
 		<Logo size="sm" />