Add frontend skeleton for plan limits

What does this MR do and why?

It adds placeholders for the plan limit UI as part of https://gitlab.com/groups/gitlab-org/-/epics/9173

Screenshots or screen recordings

before	after
N/A

How to set up and validate locally

• enable the new feature flag:

    Feature.enable(:plan_limits_admin_dashboard)

• Visit /admin/plan_limits

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.

Related to #381676