Download/save generated artifacts

### Feature summary

Add a UI option to download/save generated artifacts

### What problem are you trying to solve?

When artifacts are generated during a chat session, we can't easily save the file to another location.

### Proposed solution

Add a download/save button next to the artifact.

A "copy to clipboard" could also be useful.

### Workflow impact

This will make the workflow easier when dealing with files. Right now, you need to ask for the path where the file was created so you can copy it or select all the contents and copy/paste them (not practical with large files or binary files).

### Installation context

_No response_

### Additional context

_No response_

未指定文件、测试或入口点。首先定位用于渲染生成的工件的桌面 UI，以及现有的文件保存或剪贴板流程。完成的标准是，工件具有可见的下载/保存操作，并且该操作适用于应用支持的文件类型；复制到剪贴板是可选的后续工作。

由索引模型根据 Issue 内容生成。