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/ExplicitBadge.svelte

@@ -7,7 +7,7 @@
 </script>
 
 <span
-	class="grid h-4 w-4 shrink-0 place-items-center rounded-sm bg-fg-3 text-[11px] leading-4 font-bold text-bg {klass}"
+	class="grid h-4 w-4 shrink-0 place-items-center rounded-sm bg-fg-3 text-xs leading-4 font-bold text-bg {klass}"
 	title="Contenido explícito"
 	aria-label="Contenido explícito">E</span
 >