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/TabsBar.svelte

@@ -44,7 +44,7 @@
 				: 'text-fg-2'}"
 		>
 			<link.icon class="h-[17px] w-[17px]" strokeWidth={1.6} />
-			<span class="text-[10px] font-medium">{link.label}</span>
+			<span class="text-xs font-medium">{link.label}</span>
 		</a>
 	{/each}
 </nav>