feat(auto): browse and play local music folders in Android Auto [size:exception]

Phase 1: pick a device folder via SAF (persisted grant, no new
permission), browse its nested subfolders/tracks as a 5th Android
Auto root folder (hidden until configured), and play tracks through
the existing pipeline (EQ, art rotation, cold-start-safe source).
No metadata/sort/filter/shuffle yet -- filename is the title, generic
rotating art is the placeholder; deferred to a follow-up phase.

Adds a new pluriwave/file_actions native method (listAudioChildren)
and an onActivityResult override in MainActivity for the SAF folder
picker -- both static-review-only, no Android build available here.

lib/main.dart

@@ -6,6 +6,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:shared_preferences/shared_preferences.dart';
 import 'app.dart';
+import 'servicios/musica_local_auto.dart';
 import 'servicios/navegacion_auto.dart';
 import 'servicios/servicio_audio.dart';
 import 'servicios/servicio_audio_session.dart';
@@ -49,6 +50,13 @@ Future<void> main() async {
   final fuenteAuto = FuenteEmisorasAutoLocal();
   registrarFuenteNavegacion(fuenteAuto);
 
+  // Local-music browse source (Design "getChildren data source
+  // registration"), same injectable-prefs DI convention as every other
+  // startup service — required so `_fuenteMusicaLocalGlobal` is ever
+  // non-null; without this registration the local-music root would stay
+  // permanently hidden (`hayCarpetaConfigurada()` has no fuente to ask).
+  registrarFuenteMusicaLocal(FuenteMusicaLocalAutoImpl(prefs: prefs));
+
   // S3-R1: audio focus — phone calls / transient losses pause or duck the
   // radio; headphones unplugged pauses it.
   final sesionAudio = ServicioAudioSession(objetivo: handler);