Add budget caps scaffold: FF, routes, controllers, views, JS helpers

What does this MR do and why?

Introduces the scaffold for the per-user credit caps UI under the GitLab credits dashboard:

• Feature flag credit_caps_ui (namespace-scoped, default off, milestone 19.4)
• Routes: admin/gitlab_credits_dashboard/credit_caps and groups/settings/gitlab_credits_dashboard/credit_caps
• Controllers and HAML views (empty mount points for the Vue app)
• JS route path helpers (adminCreditCapsPath, groupSettingsCreditCapsPath)

The pages render but show no content until the Vue frontend MR lands.

References

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

Screenshots or screen recordings

SM	SaaS

How to set up and validate locally

1. Enable FF in rails c: Feature.enable(:credit_caps_ui)
2. Open page
   1. For SM, open http://localhost:3000/admin/gitlab_credits_dashboard/credit_caps
   2. For SaaS, open http://localhost:3000/groups/gitlab-org/-/settings/gitlab_credits_dashboard/credit_caps
3. Ensure that the page has the relevant html placeholders (empty divs)

MR acceptance checklist

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