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_ja.arb

@@ -656,5 +656,18 @@
   "eqDeviceNameLabel": "デバイス名",
   "eqDeviceNameHint": "例：リビングのスピーカー",
   "eqDeviceNameConfirm": "保存",
-  "eqDeviceConnected": "接続中"
+  "eqDeviceConnected": "接続中",
+  "localMusicSectionTitle": "ローカル音楽（Android Auto）",
+  "localMusicSectionDescription": "この端末のフォルダーを選択して、車内でその音声ファイルを閲覧・再生します。",
+  "localMusicFolderNotConfigured": "フォルダーが選択されていません",
+  "localMusicFolderTitle": "ローカル音楽フォルダー",
+  "localMusicChoosePath": "フォルダーを選択",
+  "localMusicChangePath": "フォルダーを変更",
+  "localMusicFolderUpdated": "ローカル音楽フォルダーを更新しました",
+  "localMusicFolderSaveError": "フォルダーを保存できませんでした: {error}",
+  "@localMusicFolderSaveError": {
+    "placeholders": {
+      "error": {}
+    }
+  }
 }