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/MediaCard.svelte

@@ -27,6 +27,6 @@
 	{@render art(mediaCardArt)}
 	<div class="mt-2.5 truncate text-sm font-medium text-fg">{title}</div>
 	{#if subtitle}
-		<div class="mt-0.5 truncate text-[12.5px] text-fg-3">{subtitle}</div>
+		<div class="mt-0.5 truncate text-xs text-fg-3">{subtitle}</div>
 	{/if}
 </a>