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)

src/app/admin/admin-users-tab.component.scss

@@ -0,0 +1,23 @@
+@use '../../styles/variables' as *;
+@use '../../styles/mixins' as *;
+
+.add-admin-section {
+  margin-top: 2rem;
+  padding: 1.5rem;
+  border: 1px solid var(--color-light, #e0e0e0);
+  border-radius: 8px;
+
+  h3 {
+    margin: 0 0 1rem 0;
+  }
+}
+
+.admin-users-section {
+  margin-bottom: 1rem;
+}
+
+.loading-text {
+  text-align: center;
+  padding: 2rem;
+  color: var(--color-medium, #888);
+}