Add One-Click "Copy" Button for Text Responses in AI Assistant Side Chat

[Blaec-CoCalc]

Is your feature request related to a problem? Please describe.
Currently, the CoCalc AI assistant side chat already provides a one-click "Copy" button for code blocks. However, there is no equivalent button for plain text responses, which makes copying text (such as explanations or answers not in code form) less convenient. Manually selecting and copying text cells is inefficient and more prone to errors.

Describe the solution you'd like
Introduce a one-click "Copy" button for text cells in the AI assistant side chat, similar to the existing button for code blocks. This will make it easier to copy non-code outputs with a single click.

Describe alternatives you've considered
Continuing to manually select and copy text cells, which is slower and less reliable. No fully satisfactory workarounds exist.

Additional context
Platforms like Gemini offer a one-click "Copy" button for all types of responses, which significantly improves user experience. Adding this feature for text cells would make CoCalc’s AI assistant more user-friendly and efficient.

No quick copy button:

Notice copy icon in Gemini (this is now my defacto user experience expectation):

[williamstein]

That's a great idea, and something I've always wanted.