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/servicios/navegacion_auto.dart

@@ -6,6 +6,7 @@ import 'package:path_provider/path_provider.dart';
 
 import '../estado/orden_emisoras.dart';
 import '../modelos/emisora.dart';
+import '../modelos/grupo_favoritos.dart';
 import 'persistencia_tolerante.dart';
 import 'servicio_favoritos.dart';
 
@@ -92,6 +93,11 @@ abstract class FuenteEmisorasAuto {
   Future<List<Emisora>> todas();
   Future<Emisora?> porUuid(String uuid);
 
+  /// Favorite groups (`GrupoFavoritos`), cold-start safe — mirrors
+  /// [favoritos]'s never-throws contract (Design "Favorite Group
+  /// Sub-Folders").
+  Future<List<GrupoFavoritos>> grupos();
+
   /// Live-snapshot push (Design "live snapshot the source prefers"):
   /// `EstadoRadio`, when alive, calls this unconditionally on every
   /// favorites/custom/populares mutation so a car and phone that are both
@@ -103,6 +109,7 @@ abstract class FuenteEmisorasAuto {
     List<Emisora>? favoritos,
     List<Emisora>? misEmisoras,
     List<Emisora>? todas,
+    List<GrupoFavoritos>? grupos,
   }) {}
 }
 
@@ -120,6 +127,16 @@ class ConstructorArbolAuto {
   static const _idsCarpetas = {idFavoritos, idTodas, idMisEmisoras};
   static const _maxItemsPorCarpeta = 50;
 
+  /// Favorite-group folder id prefix (Design "media-id scheme"), collision
+  /// free against [_prefijoEmisora] and the bare folder id constants above.
+  static const _prefijoGrupo = 'grupo:';
+
+  /// Separate cap for favorite-group folders under `Favoritos` (Design
+  /// "group-folder ordering and cap"): a folder tap costs more driver
+  /// attention than a station scroll, so this is tunable independently of
+  /// [_maxItemsPorCarpeta].
+  static const _maxGruposPorFavoritos = 50;
+
   /// Content-style extras (Design "content style", optional polish): list
   /// (1) for the root's folders, grid (2) for playable station items.
   static const _contentStyleLista = {
@@ -182,6 +199,60 @@ class ConstructorArbolAuto {
     }
     return null;
   }
+
+  /// Whether [id] identifies a favorite-group folder (Design "media-id
+  /// scheme").
+  bool esCarpetaGrupo(String id) => id.startsWith(_prefijoGrupo);
+
+  /// Maps a [GrupoFavoritos] to a non-playable folder `MediaItem` with id
+  /// `grupo:<id>` (Design "media-id scheme").
+  MediaItem itemGrupo(GrupoFavoritos g) =>
+      _carpeta('$_prefijoGrupo${g.id}', g.nombre);
+
+  /// Children of the `Favoritos` folder (Design "Ungrouped favorites stay as
+  /// direct leaves at the Favoritos root"): non-empty custom-group folders
+  /// (phone order, capped at [_maxGruposPorFavoritos]), followed by
+  /// `sin_asignar` stations mapped through the existing [hijos] path so the
+  /// no-custom-groups case is byte-identical to the pre-groups tree
+  /// (regression guard — Spec "Ungrouped station appears exactly as
+  /// before"). Empty custom groups are omitted (Design "Empty groups hidden
+  /// from the car tree"); the `sin_asignar` pseudo-group is never rendered
+  /// as its own folder.
+  List<MediaItem> carpetasFavoritos({
+    required List<GrupoFavoritos> grupos,
+    required List<Emisora> favoritos,
+  }) {
+    final carpetas = grupos
+        .where((g) => !g.esSinAsignar)
+        .where((g) => favoritos.any((e) => e.grupoFavoritosId == g.id))
+        .take(_maxGruposPorFavoritos)
+        .map(itemGrupo)
+        .toList();
+    final sinAsignar = favoritos
+        .where((e) => e.grupoFavoritosId == GrupoFavoritos.sinAsignarId)
+        .toList();
+    return [...carpetas, ...hijos(idFavoritos, emisoras: sinAsignar)];
+  }
+
+  /// Members of the favorite group identified by [grupoMediaId] (a
+  /// `grupo:<id>` id), sorted and capped like every other folder (Spec "Car
+  /// requests a group folder's stations"). An unknown/stale/malformed id
+  /// returns an empty list instead of throwing (Spec "Car requests an
+  /// unknown or stale group id").
+  List<MediaItem> hijosGrupo(
+    String grupoMediaId, {
+    required List<Emisora> favoritos,
+  }) {
+    if (!esCarpetaGrupo(grupoMediaId)) return const [];
+    final id = grupoMediaId.substring(_prefijoGrupo.length);
+    if (id.isEmpty) return const [];
+    final miembros = favoritos
+        .where((e) => e.grupoFavoritosId == id)
+        .toList();
+    if (miembros.isEmpty) return const [];
+    final ordenados = ordenarEmisoras(miembros, OrdenEmisoras.calidad);
+    return ordenados.take(_maxItemsPorCarpeta).map(itemEmisora).toList();
+  }
 }
 
 /// Routing seam between a car-tapped `emisora:<uuid>` media id and the
@@ -239,6 +310,7 @@ class FuenteEmisorasAutoLocal implements FuenteEmisorasAuto {
   List<Emisora>? _snapshotFavoritos;
   List<Emisora>? _snapshotMisEmisoras;
   List<Emisora>? _snapshotTodas;
+  List<GrupoFavoritos>? _snapshotGrupos;
 
   /// Overrides the next reads with `EstadoRadio`'s live in-memory lists
   /// (Design "live snapshot the source prefers"). Passing `null` for a
@@ -248,10 +320,12 @@ class FuenteEmisorasAutoLocal implements FuenteEmisorasAuto {
     List<Emisora>? favoritos,
     List<Emisora>? misEmisoras,
     List<Emisora>? todas,
+    List<GrupoFavoritos>? grupos,
   }) {
     if (favoritos != null) _snapshotFavoritos = favoritos;
     if (misEmisoras != null) _snapshotMisEmisoras = misEmisoras;
     if (todas != null) _snapshotTodas = todas;
+    if (grupos != null) _snapshotGrupos = grupos;
   }
 
   @override
@@ -267,6 +341,19 @@ class FuenteEmisorasAutoLocal implements FuenteEmisorasAuto {
     }
   }
 
+  @override
+  Future<List<GrupoFavoritos>> grupos() async {
+    final snapshot = _snapshotGrupos;
+    if (snapshot != null) return snapshot;
+    try {
+      return await _favoritosServicio.obtenerGrupos();
+    } catch (_) {
+      // Cold-start safety (Spec "Browse requested before app state is
+      // loaded"): never throw out of a browse call.
+      return const [];
+    }
+  }
+
   @override
   Future<List<Emisora>> misEmisoras() async {
     final snapshot = _snapshotMisEmisoras;