Show disabled fork button for user without enough permissions

What does this MR do and why?

Contributes to #259693 (closed)

Problem

We currently hide both the fork button and the fork count on the project's page when the user cannot fork the project. This behavior does not match other pages where the user can see the fork count number.

Solution

• Mark fork button as disabled
• Show tooltip with an explanation

Screenshots or screen recordings

How to set up and validate locally

1. Invite user with Guest permissions to the private project
2. Sign in as the Guest user
3. Visit project page
4. Verify that fork button and forks count are visible and disabled

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.