feat(eq): android auto custom equalizer and robust device detection

- MainActivity: onListen re-emits the current active device and registers
  the audio device callback idempotently, so recreated activities resync
  instead of freezing the active-device id on a disconnected device.
- servicio_dispositivo_audio: resubscribir() re-opens the event channel;
  estado_ecualizador exposes refrescarDispositivoActual() with an
  in-flight guard, invoked on app resume and when opening advanced EQ
  options, clearing stale green-dot device selections.
- navegacion_auto/servicio_audio: new 'Personalizado' browse tree in
  Android Auto (5 band folders, 13 gain steps each) applied live via
  setBanda; preset and gain taps persist at device level when
  multi-device EQ is active and respect station/matrix overrides,
  with apply-before-persist ordering and children-changed notifications.
- l10n: regenerate stale generated localizations; add rxdart as direct
  dependency for the subscribeToChildren override.

lib/l10n/gen/app_localizations_en.dart

@@ -1301,4 +1301,34 @@ class AppLocalizationsEn extends AppLocalizations {
 
   @override
   String get eqDeviceConnected => 'Connected';
+
+  @override
+  String get localMusicSectionTitle => 'Local music (Android Auto)';
+
+  @override
+  String get localMusicSectionDescription =>
+      'Pick a folder on this device to browse and play its audio files from the car.';
+
+  @override
+  String get localMusicFolderNotConfigured => 'No folder selected';
+
+  @override
+  String get localMusicFolderTitle => 'Local music folder';
+
+  @override
+  String get localMusicChoosePath => 'Choose folder';
+
+  @override
+  String get localMusicChangePath => 'Change folder';
+
+  @override
+  String get localMusicFolderUpdated => 'Local music folder updated';
+
+  @override
+  String localMusicFolderSaveError(Object error) {
+    return 'Could not save the folder: $error';
+  }
+
+  @override
+  String get localMusicFolderGenericName => 'Selected folder';
 }