Frontend web page, upload files can not save to current work dir

**Version:**
aioncore-v0.1.30

**problem:**
User uploads a file (by dragging and dropping, pasting, or clicking the attachment button)
│
▼
front-end call POST /api/fs/upload（multipart）
│
▼
backend FileService::create_upload_file()
│
▼
❌ Hard coding uses std::env::temp_dir() as the storage path
│ → /aionui//
│ → or /aionui/general/

**Expect:**
Upload files and save them to the current working directory

**Actual**:
Uploaded file save to /aionui//

Start at backend FileService::create_upload_file() and trace the POST /api/fs/upload multipart handler, including how the current working directory is obtained. Verify drag-and-drop, paste, and attachment-button uploads for both conversation and general files, and confirm they are saved under the working directory rather than std::env::temp_dir().

Written by the indexing model from the issue text.