feat(auto): surface favorite groups as Android Auto sub-folders

Favoritos now renders non-empty custom groups as grupo:<id>
sub-folders (hidden when empty) with ungrouped stations left as
direct leaves, reusing the existing hijos() path so the zero-groups
case stays byte-identical to today's flat list.

lib/estado/estado_radio.dart

@@ -345,6 +345,9 @@ class EstadoRadio extends ChangeNotifier {
 
   Future<void> cargarGruposFavoritos() async {
     _gruposFavoritos = await favoritos.obtenerGrupos();
+    // Design "live snapshot the source prefers": Android Auto's grouped
+    // Favoritos tree mirrors the same group list the phone just loaded.
+    _fuenteAuto?.actualizarSnapshot(grupos: _gruposFavoritos);
     notifyListeners();
   }