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_

No files, tests, or entry points are named. Start by locating the desktop UI that renders generated artifacts and any existing file-saving or clipboard flows. Done means an artifact has a visible download/save action that works for the file types supported by the app; clipboard copying is an optional follow-up.

索引モデルが issue の本文から書いたものです。