[FE-64] Build /admin/activity page

## Overview
No admin activity-timeline UI exists yet.

## Depends on
BE-92 (activity tracker endpoint), FE-42, FE-44, FE-45

## Acceptance Criteria
- [ ] Paginated user-action timeline from `GET /admin/activity` with `userId` and `actionType` filters
- [ ] Grouped/sorted by most recent first
- [ ] Admin-only, guarded per FE-44