support default file pool selection

- add defaultPoolId to AppSettings + persistence
- extend SettingsScreen with pool dropdown
- update uploadAttachment to use defaultPoolId with fallback

Signed-off-by: Texas0295 <kimura@texas0295.top>

assets/i18n/zh-CN.json

@@ -123,6 +123,8 @@
   "addPhoto": "添加照片",
   "addFile": "添加文件",
   "uploadFile": "上传文件",
+  "settingsDefaultPool": "选择文件池",
+  "settingsDefaultPoolHelper": "为文件上传选择一个默认池",
   "createDirectMessage": "创建新私人消息",
   "gotoDirectMessage": "前往私信",
   "react": "反应",