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

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

@@ -2,25 +2,25 @@
 	import type { Snippet } from 'svelte';
 
 	interface Props {
-		label: string;
+		label?: string;
 		for?: string;
-		optional?: boolean;
 		class?: string;
 		hint?: Snippet;
 		children: Snippet;
 	}
 
-	let { label, for: forId, optional = false, class: klass = '', hint, children }: Props = $props();
+	let { label, for: forId, class: klass = '', hint, children }: Props = $props();
 </script>
 
 <div class="flex min-w-0 flex-col {klass}">
-	<label for={forId} class="mb-2 block text-base font-medium text-fg-2">
-		{label}
-		{#if optional}<span class="text-muted">(Opcional)</span>{/if}
-	</label>
+	{#if label}
+		<label for={forId} class="mb-2 block text-sm font-medium text-fg-2">
+			{label}
+		</label>
+	{/if}
 	{@render children()}
 	{#if hint}
-		<div class="mt-2 flex items-center justify-between gap-3 text-sm text-muted">
+		<div class="mt-2 flex items-center justify-between gap-3 text-xs text-muted">
 			{@render hint()}
 		</div>
 	{/if}