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_pt.arb

@@ -656,5 +656,18 @@
   "eqDeviceNameLabel": "Nome do dispositivo",
   "eqDeviceNameHint": "Ex: Caixa da sala",
   "eqDeviceNameConfirm": "Salvar",
-  "eqDeviceConnected": "Conectado"
+  "eqDeviceConnected": "Conectado",
+  "localMusicSectionTitle": "Música local (Android Auto)",
+  "localMusicSectionDescription": "Escolha uma pasta neste dispositivo para navegar e reproduzir os arquivos de áudio dela no carro.",
+  "localMusicFolderNotConfigured": "Nenhuma pasta selecionada",
+  "localMusicFolderTitle": "Pasta de música local",
+  "localMusicChoosePath": "Escolher pasta",
+  "localMusicChangePath": "Alterar pasta",
+  "localMusicFolderUpdated": "Pasta de música local atualizada",
+  "localMusicFolderSaveError": "Não foi possível salvar a pasta: {error}",
+  "@localMusicFolderSaveError": {
+    "placeholders": {
+      "error": {}
+    }
+  }
 }