Feature: 资源管理器支持文件树中拖放功能

具体包含两个功能：

- [ ] 文件树中已经存在的文件或目录拖放到其他目录下，体现为文件的移动；按住 Ctrl 时进行拖动体现为文件的复制
- [ ] 外部文件拖动到浏览器窗口内的文件树区域时，体现为文件上传到所在区域对应的目录中

The issue does not name files or tests. Start by locating the file-tree entry point and its existing drag-and-drop handling, then trace how file moves, copies, and uploads are performed. Done means supporting internal moves, Ctrl-drag copies, and external uploads into the directory under the drop target.

Written by the indexing model from the issue text.