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_en.arb

@@ -656,5 +656,18 @@
   "eqDeviceNameLabel": "Device name",
   "eqDeviceNameHint": "e.g. Living Room Speaker",
   "eqDeviceNameConfirm": "Save",
-  "eqDeviceConnected": "Connected"
+  "eqDeviceConnected": "Connected",
+  "localMusicSectionTitle": "Local music (Android Auto)",
+  "localMusicSectionDescription": "Pick a folder on this device to browse and play its audio files from the car.",
+  "localMusicFolderNotConfigured": "No folder selected",
+  "localMusicFolderTitle": "Local music folder",
+  "localMusicChoosePath": "Choose folder",
+  "localMusicChangePath": "Change folder",
+  "localMusicFolderUpdated": "Local music folder updated",
+  "localMusicFolderSaveError": "Could not save the folder: {error}",
+  "@localMusicFolderSaveError": {
+    "placeholders": {
+      "error": {}
+    }
+  }
 }