feat: admin panel with user table and stats (Task 12)

- AdminPage: password gate (sessionStorage), user table, stats banner
- Auto-login using stored sessionStorage password on mount
- App.tsx: route to admin when pathname === '/admin'
- Fix: remove redundant setPassword in auto-login effect (useState already seeds from sessionStorage)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/src/App.tsx

@@ -3,6 +3,7 @@ import { useAuth } from './hooks/useAuth'
 import { LoginPage } from './pages/LoginPage'
 import { OnboardingPage } from './pages/OnboardingPage'
 import { GamePage } from './pages/GamePage'
+import { AdminPage } from './pages/AdminPage'
 import './index.css'
 
 type AppRoute = 'login' | 'sent' | 'onboarding' | 'game' | 'admin'
@@ -12,6 +13,13 @@ export default function App() {
   const [route, setRoute] = useState<AppRoute>('login')
   const [checkingOnboarding, setCheckingOnboarding] = useState(false)
 
+  // Check for /admin path
+  useEffect(() => {
+    if (window.location.pathname === '/admin') {
+      setRoute('admin')
+    }
+  }, [])
+
   // After auth resolves, check onboarding status
   useEffect(() => {
     if (!profile || !token) return
@@ -55,12 +63,7 @@ export default function App() {
       {route === 'game' && token && (
         <GamePage token={token} onLogout={() => { logout(); setRoute('login') }} />
       )}
-      {route === 'admin' && (
-        <div>
-          <h1 style={{ fontFamily: 'var(--font-header)' }}>Admin</h1>
-          <p>Admin panel coming in Task 12.</p>
-        </div>
-      )}
+      {route === 'admin' && <AdminPage />}
     </div>
   )
 }