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/EmptyState.svelte

@@ -23,7 +23,7 @@
 		</h3>
 	{/if}
 	{#if description}
-		<p class="mt-2.5 max-w-sm text-[13px] leading-[1.65] text-fg-2">{description}</p>
+		<p class="mt-2.5 max-w-sm text-sm leading-[1.65] text-fg-2">{description}</p>
 	{/if}
 	{#if actions}
 		<div class="mt-6 flex flex-wrap items-center justify-center gap-2.5">{@render actions()}</div>