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/routes/+error.svelte

@@ -7,7 +7,7 @@
 	<h1 class="mb-2.5 font-display text-xl font-semibold tracking-[-0.02em] text-fg">
 		{page.status === 404 ? 'Página no encontrada' : 'No se pudo cargar tu música'}
 	</h1>
-	<p class="mb-6.5 text-[13px] leading-[1.65] text-fg-2">
+	<p class="mb-6.5 text-sm leading-[1.65] text-fg-2">
 		{page.status === 404
 			? 'Comprueba el enlace o vuelve al inicio.'
 			: 'La conexión se interrumpió. Lo que tengas descargado seguirá disponible sin conexión.'}