test: add data-testid attributes to admin sub-components and modals

frontend/components/admin/IdentityManager.tsx

@@ -34,6 +34,7 @@ export default function IdentityManager({
         </div>
         <button
           onClick={onAddUser}
+          data-testid="add-user-button"
           className="flex items-center gap-2 px-5 py-2.5 bg-primary hover:bg-blue-600 text-white rounded-xl text-sm font-black transition-all shadow-xl shadow-primary/10 active:scale-95 border border-primary/30 tracking-tight focus-visible:ring-2 focus-visible:ring-primary focus-visible:outline-none"
           aria-label="Add new user"
         >
@@ -70,6 +71,7 @@ export default function IdentityManager({
               {user.username !== 'Admin' && (
                 <button
                   onClick={() => onDeleteUser(user.id, user.username)}
+                  data-testid="delete-user-button"
                   className="p-2 text-secondary hover:text-rose-500 hover:bg-rose-500/5 rounded-xl transition-all focus-visible:ring-2 focus-visible:ring-rose-500 focus-visible:outline-none"
                   aria-label={`Delete user ${user.username}`}
                 >