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/api/likes/+server.ts

@@ -0,0 +1,16 @@
+import type { RequestHandler } from './$types';
+import { json } from '@sveltejs/kit';
+import { createApiClient } from '$lib/server/api';
+
+export const POST: RequestHandler = async ({ request, locals, fetch }) => {
+	if (!locals.accessToken) return json({ message: 'Inicia sesión.' }, { status: 401 });
+
+	const body = await request.json();
+	const api = createApiClient({ fetch, accessToken: locals.accessToken });
+	const result = await api.POST('/likes/toggle', { body });
+
+	if (result.error || result.data === undefined)
+		return json({ message: 'No se pudo actualizar Me gusta.' }, { status: 502 });
+
+	return json({ liked: result.data });
+};