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/TrackTable.svelte

@@ -56,7 +56,7 @@
 
 <div class="overflow-x-auto {klass}">
 	<div
-		class="grid track-grid items-center gap-3 border-b border-white/5 pr-4 pb-3 pl-2 text-[10.5px] font-semibold tracking-[0.12em] text-muted uppercase sm:gap-8 sm:pr-6 sm:pl-3"
+		class="grid track-grid items-center gap-3 border-b border-line pr-4 pb-3 pl-2 text-xs font-semibold tracking-[0.12em] text-muted uppercase sm:gap-8 sm:pr-6 sm:pl-3"
 		style="--mf-track-cols:{columns}; --mf-track-cols-mobile:{columnsMobile}"
 	>
 		{#if index}