Add anonymous listening support and redesign web-player UI

Backend:
- Expose GET /config/playback so the frontend can detect whether
  anonymous listening is enabled without requiring auth
- Wire anonymous access through track streaming and YouTube resolve flows

Frontend:
- Allow browsing/listening without a session when the backend enables it
- Redesign Inicio, Sidebar and top bar with a new dark theme, typography,
  and layout (top bar with live quick-search, sticky/blurred on scroll)
- Populate Top música, Popular and Recomendados with real Musify +
  YouTube Music data instead of placeholders
- Various shape, spacing, color and animation refinements across the
  redesigned components

web-player/src/routes/(app)/explore/+page.server.ts

@@ -1,5 +1,5 @@
 import type { PageServerLoad, Actions } from './$types';
-import { createApiClient, requireUser, unwrapOrError } from '$lib/server/api';
+import { createApiClient, unwrapOrError } from '$lib/server/api';
 import { fetchYoutubeFiller } from '$lib/server/youtube';
 import {
 	addAlbumToPlaylistAction,
@@ -18,16 +18,20 @@ export const load: PageServerLoad = async ({ url, locals, fetch }) => {
 	const query = url.searchParams.get('q')?.trim() ?? '';
 	const page = Math.max(1, Number(url.searchParams.get('page')) || 1);
 
-	const user = requireUser(locals, url);
+	// This page is reachable without a session when anonymous listening is on
+	// (see +layout.server.ts), so `user` may be null here.
+	const user = locals.user;
 	const api = createApiClient({ fetch, accessToken: locals.accessToken ?? undefined });
 
 	const tracksPromise = api.GET('/tracks', {
 		params: { query: { name: query || undefined, pageNumber: page, pageSize: PAGE_SIZE } }
 	});
 
-	const playlistsPromise = api.GET('/playlists/users/{userId}', {
-		params: { path: { userId: user.sub }, query: { pageNumber: 1, pageSize: 50 } }
-	});
+	const playlistsPromise = user
+		? api.GET('/playlists/users/{userId}', {
+				params: { path: { userId: user.sub }, query: { pageNumber: 1, pageSize: 50 } }
+			})
+		: Promise.resolve(null);
 
 	const albumsPromise = query
 		? api.GET('/albums', {