Add per-user cap override modal with dual-mode user picker

What does this MR do and why?

Adds a user picker for per-user credit cap overrides modal

MR series

# Click to sort ascending	MR Click to sort ascending	Status Click to sort ascending
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	✅ Merged
5	Add override modal wired to UpsertUserBudgetCapOverrides mutation	✅ Merged
6	Add per-user cap override modal with dual-mode user picker	➡️ This MR

References

• Part of https://gitlab.com/gitlab-org/gitlab/-/work_items/596232
• Related to https://gitlab.com/gitlab-org/gitlab/-/work_items/624327

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Navigate to the credit caps dashboard
2. Click "Add override" in the "Per-user cap overrides" card header
3. The modal opens with a user search field, cap input, and status toggle
4. On SM: typing 3+ characters searches all instance users
5. On SaaS (with namespacePath): typing 3+ characters searches group members

MR acceptance checklist

• This MR does not introduce a~security vulnerability
• No documentation required