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.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

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

@@ -13,11 +13,11 @@
 
 <div class="flex flex-col gap-4 sm:flex-row sm:items-end sm:justify-between {klass}">
 	<div class="min-w-0">
-		<h1 class="font-display text-[28px] font-semibold tracking-[-0.03em] text-fg sm:text-[32px]">
+		<h1 class="font-display text-3xl font-semibold tracking-[-0.03em] text-fg sm:text-4xl">
 			{title}
 		</h1>
 		{#if subtitle}
-			<p class="mt-2 text-[13px] text-fg-2">{subtitle}</p>
+			<p class="mt-2 text-sm text-fg-2">{subtitle}</p>
 		{/if}
 	</div>
 	{#if actions}