Add likes, follows and playlist visibility; redesign web-player UI

Backend:
- New Likes feature (toggle/get liked tracks) and endpoints
- User follow/unfollow, listening stats, public user profile
- Playlist visibility (public/private) support
- Various auth and DTO/response adjustments to support the above

Frontend (web-player):
- New design system pass: theme tokens/mode (light/dark), glass menu,
  tabs bar, player dock/extras/queue components, mix tiles, artist link
- New routes: Liked, public user profile (/u/[id]), likes API proxy
- Search refactor: shared SearchBox component used by TopBar and
  MobileHeader (search now reachable on mobile), live results as you
  type instead of requiring Enter, dropped the "Buscando…" quick-result
  preview
- Removed now-unused components (MobileNav, CoverBadge, MixBentoTile,
  PlayPauseOverlay, SuggestionsTable, userDirectory)
- Design handoff reference docs added under web-player/design_handoff_musify_player
  and web-player/docs/design-system-refactor-plan.md

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web-player/src/lib/components/ui/PlaylistCard.svelte

@@ -5,16 +5,18 @@
 	interface Props {
 		id: string;
 		name: string;
-		description?: string | null;
+		trackCount: number;
 		trackIds: (string | number)[];
 		updatedAt: string;
 		index?: number;
 	}
 
-	let { id, name, description, trackIds, updatedAt, index = 0 }: Props = $props();
+	let { id, name, trackCount, trackIds, updatedAt, index = 0 }: Props = $props();
+
+	const subtitle = $derived(`${trackCount} ${trackCount === 1 ? 'canción' : 'canciones'}`);
 </script>
 
-<MediaCard href="/playlists/{id}" title={name} subtitle={description} {index}>
+<MediaCard href="/playlists/{id}" title={name} {subtitle} {index}>
 	{#snippet art(artClass)}
 		<PlaylistArt playlistId={id} {trackIds} version={updatedAt} class={artClass} />
 	{/snippet}