Add admin user management API and frontend UI

Backend:
- /api/admin/users: GET list, POST create, PUT update, DELETE
- Admin users can be added with email, display name, and password
- Self-deletion and last-admin deletion are blocked
- Admin role auto-assigned on creation

Frontend:
- AdminUsersTabComponent: list + add/delete admin users
- AdminUserService: HTTP client for admin user CRUD
- AdminUser interface
- 'Admins' tab added to admin dashboard

Tests:
- 10 new tests for admin user management endpoints
- 140 total tests pass (130 original + 10 new)

backend/app/main.py

@@ -234,6 +234,10 @@ app.include_router(stats.router, prefix="/api/stats", tags=["stats"])
 app.include_router(mobile_builds.router, prefix="/api/mobile-builds", tags=["mobile-builds"])
 app.include_router(theme.router, prefix="/api/theme", tags=["theme"])
 
+# Admin user management (available in all environments)
+from app.api import users
+app.include_router(users.router, prefix="/api/admin", tags=["admin-users"])
+
 # Dev-only admin router (disabled in production)
 if settings.ENV != "production":
     from app.api import admin