Add override modal UI

What does this MR do and why?

Adds the "Add override" modal UI to the per-user cap overrides card. Admins can open a modal form to add a new per-user AI credit cap override without leaving the page.

MR series

#	MR	Status
1	Add UserOverridesList component with data fetching and pagination	✅ Merged
2	Replace JSON output with GlTableLite in UserOverridesList	✅ Merged
3	Add Save and Remove mutation wiring to UserOverridesList	✅ Merged
4	Add override modal UI	➡️ This MR

References

• Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/596232

Screenshots or screen recordings

Before	After
N/A

How to set up and validate locally

Prerequisites: self-managed GitLab instance with the Credits Dashboard enabled.

1. Enable SM instance.
2. Open http://localhost:3000/admin/gitlab_credits_dashboard/credit_caps.
3. Click the Add override button.
4. Verify the modal opens with a user input field and a cap/enabled inputs.
5. Verify:
   • On save the modal closes
   • On cancel the modal closes
   • On open the form is cleared

Alternatively, via Storybook:

1. Start Storybook: yarn storybook:start
2. Verify the modal story renders and the form controls are functional.

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.

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Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/624327