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/routes/+error.svelte

@@ -0,0 +1,16 @@
+<script lang="ts">
+	import { page } from '$app/state';
+	import Button from '$lib/components/ui/Button.svelte';
+</script>
+
+<div class="animate-enter mx-auto my-[14vh] max-w-105 text-center">
+	<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">
+		{page.status === 404
+			? 'Comprueba el enlace o vuelve al inicio.'
+			: 'La conexión se interrumpió. Lo que tengas descargado seguirá disponible sin conexión.'}
+	</p>
+	<Button href="/" variant="secondary">Volver</Button>
+</div>