收敛移动端上传与文件能力

[yeyixiang2007]

What to build

Adapt the upload and file-handling flows so they work consistently in the native app experience, including duplicate checking, upload token flow, and file submission.
This slice should keep upload behavior aligned with the existing backend contracts while accounting for mobile device constraints.

Acceptance criteria

• The mobile app can select and upload files through the existing backend flow.
• Duplicate checking and upload token acquisition work end to end on mobile.
• Upload errors are surfaced clearly to the user.

Blocked by

• 提取共享 API 和类型层 #234
• 打包 Android 和 iOS App 壳 #235