Add ability to copy the selected ref from ref selector

What does this MR do and why?

Adds a copy button to the selected ref header.

References

Screenshots or screen recordings

Before	After

(*clipboard empty* is mock data I added to my clipboard for testing purposes)

How to set up and validate locally

1. Open the ref selector dropdown in the repository
2. A copy button should be available in the "Selected" section
3. Click the copy button
4. The selected ref should be copied to the clipboard
5. A toast should appear

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #518021 (closed)