Unify web-player design tokens and componentize collection headers

Migrates remaining one-off colors (bg-white/[N%], border-white/N, hardcoded
shadows/gradients) to the existing --mf-* tokens so light mode reacts
consistently everywhere, including a fix to --color-hover which was
hardcoded instead of pointing at --mf-surface-2. Extracts CollectionHeader
(album/playlist/mix/liked headers) and AccountMenu (TopBar/MobileHeader)
to remove five and two duplicated implementations respectively, and
normalizes inconsistent empty-state copy.

web-player/src/lib/components/ui/MenuItem.svelte

@@ -24,7 +24,7 @@
 	}: Props = $props();
 
 	const classes = $derived(
-		`flex w-full items-center gap-3 rounded-control px-2 py-2.5 text-left text-[13.5px] font-medium text-fg-2 transition hover:bg-hover ${klass}`
+		`flex w-full items-center gap-3 rounded-control px-2 py-2.5 text-left text-sm font-medium text-fg-2 transition hover:bg-hover ${klass}`
 	);
 </script>