Add user management page and technician picker for reassign

- New /admin/users page (admin only) for creating technicians,
  editing role/email, resetting passwords, deleting users
- AftersalesDetail reassign modal now uses a searchable Select
  populated from /api/users/assignable instead of raw user ID input
- Menu entry only shown to admins

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/App.tsx

@@ -10,6 +10,7 @@ import EmployeeSerialsPage from './pages/EmployeeSerials';
 import AftersalesPage from './pages/Aftersales';
 import AftersalesDetailPage from './pages/AftersalesDetail';
 import AftersalesConfirmPage from './pages/AftersalesConfirm';
+import UsersPage from './pages/Users';
 
 const PrivateRoute = () => {
   const user = authApi.getCurrentUser();
@@ -52,6 +53,7 @@ function App() {
             <Route path="/admin/employee-serials" element={<EmployeeSerialsPage />} />
             <Route path="/admin/aftersales" element={<AftersalesPage />} />
             <Route path="/admin/aftersales/:serialNumber" element={<AftersalesDetailPage />} />
+            <Route path="/admin/users" element={<UsersPage />} />
             <Route path="/admin/profile" element={<ProfilePage />} />
           </Route>
         </Route>