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/TrackContextMenu.svelte

@@ -53,7 +53,7 @@
 	<button
 		type="button"
 		onclick={onAddToQueue}
-		class="flex w-full items-center gap-3 rounded-control px-2 py-2.5 text-left text-[13.5px] font-medium text-fg-2 transition hover:bg-hover"
+		class="flex w-full items-center gap-3 rounded-control px-2 py-2.5 text-left text-sm font-medium text-fg-2 transition hover:bg-hover"
 	>
 		<ListPlus class="h-[18px] w-[18px] shrink-0" strokeWidth={2} />
 		<span>Reproducir a continuación</span>
@@ -61,7 +61,7 @@
 	{#if playlists.length > 0}
 		<div class="group/addmenu relative">
 			<div
-				class="flex items-center gap-3 rounded-control px-2 py-2.5 text-[13.5px] font-medium text-fg-2 transition group-hover/addmenu:bg-hover"
+				class="flex items-center gap-3 rounded-control px-2 py-2.5 text-sm font-medium text-fg-2 transition group-hover/addmenu:bg-hover"
 			>
 				<span class="flex-1 cursor-default">Añadir a una playlist</span>
 				<ChevronRight class="h-4 w-4 shrink-0 text-fg-3" />
@@ -97,7 +97,7 @@
 								{/if}
 								<button
 									type="submit"
-									class="flex w-full items-center gap-3 rounded-control px-2 py-2.5 text-left text-[13.5px] font-medium text-fg transition hover:bg-hover"
+									class="flex w-full items-center gap-3 rounded-control px-2 py-2.5 text-left text-sm font-medium text-fg transition hover:bg-hover"
 								>
 									<PlaylistArt
 										playlistId={playlist.id}