Fix buttons on fork modal

What does this MR do and why?

The modal prompting a user to fork a project if they do not have edit permissions but want to edit a file should use viewport-based queries for the layout of its buttons.

References

• https://gitlab.com/gitlab-org/gitlab/-/issues/567702

Screenshots or screen recordings

Before	After
Screen Recording 2025-09-25 at 17.10.57.mov	Screen Recording 2025-09-25 at 17.07.56.mov

How to set up and validate locally

1. Check out this branch
2. In the GDK, impersonate a user that does not have edit access to a project
3. Try to edit a file in that project as that user
4. You will see the "Fork to make changes" modal and it will have correctly laid out buttons.

MR acceptance checklist

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