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/Button.svelte

@@ -29,12 +29,12 @@
 	}: Props = $props();
 
 	const sizes = {
-		sm: 'px-4 py-2.5 text-[12.5px]',
-		md: 'px-5 py-3 text-[13px]',
+		sm: 'px-4 py-2.5 text-xs',
+		md: 'px-5 py-3 text-sm',
 		lg: 'px-6 py-3.5 text-sm'
 	};
 
-	const subtleButton = 'bg-white/[6%] text-fg-2 hover:bg-white/[9%] hover:text-fg';
+	const subtleButton = 'bg-surface-2 text-fg-2 hover:bg-surface-hover hover:text-fg';
 
 	const variants = {
 		primary: subtleButton,