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/lib/server/api.ts

@@ -23,9 +23,15 @@ export function requireUser(locals: App.Locals, url: URL): SessionUser {
 	return locals.user;
 }
 
-export function requireAccessToken(locals: App.Locals, message = 'Inicia sesión.'): string {
-	if (!locals.accessToken) error(401, message);
-	return locals.accessToken;
+export function optionalUser(
+	locals: App.Locals,
+	url: URL,
+	allowAnonymousListening: boolean
+): SessionUser | null {
+	if (locals.user) return locals.user;
+	if (!allowAnonymousListening)
+		redirect(302, `/login?returnTo=${encodeURIComponent(url.pathname + url.search)}`);
+	return null;
 }
 
 export function requireAccessTokenAction(