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/TrackRow.svelte

@@ -19,7 +19,7 @@
 <div
 	class="group grid track-grid items-center gap-3 rounded-control py-3 pr-4 pl-2 transition-colors sm:gap-8 sm:pr-6 sm:pl-3 {active
 		? 'bg-accent-tint'
-		: 'hover:bg-white/[3.2%]'} {klass}"
+		: 'hover:bg-hover'} {klass}"
 	style="--mf-track-cols:{table.columns}; --mf-track-cols-mobile:{table.columnsMobile}"
 	{oncontextmenu}
 >