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/pantallas/pantalla_ajustes.dart

@@ -18,6 +18,7 @@ import '../l10n/gen/app_localizations.dart';
 import '../modelos/emisora.dart';
 import '../modelos/grupo_favoritos.dart';
 import '../modelos/preset_ecualizador.dart';
+import '../servicios/musica_local_auto.dart';
 import '../widgets/ecualizador_widget.dart';
 import '../widgets/pluri_glass_surface.dart';
 import '../widgets/pluri_icon.dart';
@@ -66,6 +67,8 @@ class _AjustesContent extends StatelessWidget {
         SizedBox(height: 12),
         _SeccionGrabaciones(),
         SizedBox(height: 12),
+        _SeccionMusicaLocal(),
+        SizedBox(height: 12),
         _SeccionTimerSueno(),
         SizedBox(height: 12),
         _SeccionIdioma(),
@@ -278,6 +281,118 @@ class _SeccionGrabaciones extends StatelessWidget {
   }
 }
 
+/// Local-music root-folder picker (android-auto-local-music task 9),
+/// mirroring [_SeccionGrabaciones]'s shape: `PluriGlassSurface` card,
+/// `FutureBuilder`-driven current-folder display, a single action button and
+/// snackbar feedback. Deliberately does NOT use `FilePicker.platform` (see
+/// tasks.md "Grounding corrections") — [FuenteMusicaLocalAutoImpl.elegirCarpeta]
+/// calls the NEW native `pickMusicFolder` channel method directly, since it
+/// needs a persistable-grant SAF tree URI, not a plain filesystem path.
+class _SeccionMusicaLocal extends StatefulWidget {
+  const _SeccionMusicaLocal();
+
+  @override
+  State<_SeccionMusicaLocal> createState() => _SeccionMusicaLocalState();
+}
+
+class _SeccionMusicaLocalState extends State<_SeccionMusicaLocal> {
+  final _fuente = FuenteMusicaLocalAutoImpl();
+  late Future<String?> _carpetaActual;
+
+  @override
+  void initState() {
+    super.initState();
+    _carpetaActual = _fuente.carpetaActual();
+  }
+
+  Future<void> _elegirCarpeta(BuildContext context) async {
+    final messenger = ScaffoldMessenger.of(context);
+    final l10n = AppLocalizations.of(context);
+    try {
+      final uri = await _fuente.elegirCarpeta();
+      if (uri == null) return; // Cancelled — no snackbar, matches the SAF
+      // picker's own "nothing changed" affordance.
+      if (!context.mounted) return;
+      setState(() {
+        _carpetaActual = Future.value(uri);
+      });
+      messenger.showSnackBar(
+        SnackBar(content: Text(l10n.localMusicFolderUpdated)),
+      );
+    } catch (e) {
+      if (!context.mounted) return;
+      messenger.showSnackBar(
+        SnackBar(content: Text(l10n.localMusicFolderSaveError(e.toString()))),
+      );
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final l10n = AppLocalizations.of(context);
+
+    return PluriGlassSurface(
+      child: Column(
+        crossAxisAlignment: CrossAxisAlignment.start,
+        children: [
+          Row(
+            children: [
+              const Icon(Icons.library_music_outlined),
+              const SizedBox(width: 12),
+              Text(
+                l10n.localMusicSectionTitle,
+                style: Theme.of(context).textTheme.titleMedium,
+              ),
+            ],
+          ),
+          const SizedBox(height: 8),
+          Text(
+            l10n.localMusicSectionDescription,
+            style: Theme.of(context).textTheme.bodySmall,
+          ),
+          FutureBuilder<String?>(
+            future: _carpetaActual,
+            builder: (ctx, snap) {
+              final carpeta = snap.data;
+              return ListTile(
+                contentPadding: EdgeInsets.zero,
+                leading: const Icon(Icons.folder_outlined),
+                title: Text(l10n.localMusicFolderTitle),
+                subtitle: Text(
+                  (carpeta == null || carpeta.isEmpty)
+                      ? l10n.localMusicFolderNotConfigured
+                      : carpeta,
+                  maxLines: 2,
+                  overflow: TextOverflow.ellipsis,
+                ),
+              );
+            },
+          ),
+          const SizedBox(height: 8),
+          FutureBuilder<String?>(
+            future: _carpetaActual,
+            builder: (ctx, snap) {
+              final configurada = (snap.data ?? '').isNotEmpty;
+              return Align(
+                alignment: Alignment.centerLeft,
+                child: OutlinedButton.icon(
+                  icon: const Icon(Icons.folder_open_rounded),
+                  label: Text(
+                    configurada
+                        ? l10n.localMusicChangePath
+                        : l10n.localMusicChoosePath,
+                  ),
+                  onPressed: () => _elegirCarpeta(context),
+                ),
+              );
+            },
+          ),
+        ],
+      ),
+    );
+  }
+}
+
 class _SeccionTimerSueno extends StatelessWidget {
   const _SeccionTimerSueno();