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_zh.arb

@@ -656,5 +656,18 @@
   "eqDeviceNameLabel": "设备名称",
   "eqDeviceNameHint": "例：客厅音箱",
   "eqDeviceNameConfirm": "保存",
-  "eqDeviceConnected": "已连接"
+  "eqDeviceConnected": "已连接",
+  "localMusicSectionTitle": "本地音乐（Android Auto）",
+  "localMusicSectionDescription": "选择此设备上的一个文件夹，以便在车内浏览和播放其中的音频文件。",
+  "localMusicFolderNotConfigured": "未选择文件夹",
+  "localMusicFolderTitle": "本地音乐文件夹",
+  "localMusicChoosePath": "选择文件夹",
+  "localMusicChangePath": "更改文件夹",
+  "localMusicFolderUpdated": "本地音乐文件夹已更新",
+  "localMusicFolderSaveError": "无法保存文件夹：{error}",
+  "@localMusicFolderSaveError": {
+    "placeholders": {
+      "error": {}
+    }
+  }
 }