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/l10n/app_es.arb

@@ -619,5 +619,18 @@
   "eqDeviceNameLabel": "Nombre del dispositivo",
   "eqDeviceNameHint": "Ej: Altavoz del living",
   "eqDeviceNameConfirm": "Guardar",
-  "eqDeviceConnected": "Conectado"
+  "eqDeviceConnected": "Conectado",
+  "localMusicSectionTitle": "Música local (Android Auto)",
+  "localMusicSectionDescription": "Elegí una carpeta de este dispositivo para explorar y reproducir sus archivos de audio desde el auto.",
+  "localMusicFolderNotConfigured": "No hay carpeta seleccionada",
+  "localMusicFolderTitle": "Carpeta de música local",
+  "localMusicChoosePath": "Elegir carpeta",
+  "localMusicChangePath": "Cambiar carpeta",
+  "localMusicFolderUpdated": "Carpeta de música local actualizada",
+  "localMusicFolderSaveError": "No se pudo guardar la carpeta: {error}",
+  "@localMusicFolderSaveError": {
+    "placeholders": {
+      "error": {}
+    }
+  }
 }