Add copy URL button to merge request header

Problem

Currently, there's no easy way to copy a merge request's full URL. The "Copy reference" option in the three-dot menu only copies the reference (e.g., !15979), not the full URL.

Proposed Solution

Add a "Copy URL" button in the merge request header (near the title or in the actions menu) that copies the full merge request URL to the clipboard.

Why This Matters

• Users frequently need to share MR links in chat, emails, and documentation
• Currently requires manually selecting the address bar and copying
• Other GitLab resources (issues, epics) could benefit from the same feature

Example

A small copy icon button next to the MR title or in the three-dot menu labeled "Copy URL" that copies https://gitlab.com/safecorp/employees-of-safecorp-technology-inc/services/mercury/-/merge_requests/15979