feat: add admin Dashboard, Users, and Activity Log pages

- Add backend routes: GET /api/admin/users, /api/admin/users/:id,
  POST /api/admin/users/:id/toggle-admin, DELETE /api/admin/users/:id,
  GET /api/admin/stats, GET /api/admin/activity
- Add frontend Dashboard page with stat cards (total users/servers/etc.)
- Add frontend Users list page with admin badge and server counts
- Add frontend User detail page with info, IPs, servers, toggle-admin, delete
- Add frontend Activity Log page showing all platform activity
- Update admin navbar with 4 tabs: Dashboard, Servers, Users, Activity
- Update routing/navigation for all new pages

Author: ddrayko