feat(phase5): Add admin panel with JWT auth, questions CRUD, and calendar

- Add adminStore with Zustand for authentication state persistence
- Add adminApi service for all admin endpoints
- Add Login page with error handling and redirect
- Add AdminLayout with sidebar navigation and route protection
- Add Dashboard with stats and quick actions
- Add Questions page with full CRUD, filters, and AI generation modal
- Add Calendar page for scheduling questions by date
- Integrate admin routes in App.tsx with nested routing

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,19 +1,30 @@
-import { Routes, Route } from 'react-router-dom'
+import { Routes, Route, Navigate } from 'react-router-dom'
 import Home from './pages/Home'
 import Lobby from './pages/Lobby'
 import Game from './pages/Game'
 import Results from './pages/Results'
 import Replay from './pages/Replay'
+import { AdminLayout, Login, Dashboard, Questions, Calendar } from './pages/admin'
 
 function App() {
   return (
     <div className="min-h-screen">
       <Routes>
+        {/* Game routes */}
         <Route path="/" element={<Home />} />
         <Route path="/lobby/:roomCode" element={<Lobby />} />
         <Route path="/game/:roomCode" element={<Game />} />
         <Route path="/results/:roomCode" element={<Results />} />
         <Route path="/replay/:replayCode" element={<Replay />} />
+
+        {/* Admin routes */}
+        <Route path="/admin/login" element={<Login />} />
+        <Route path="/admin" element={<AdminLayout />}>
+          <Route index element={<Navigate to="dashboard" replace />} />
+          <Route path="dashboard" element={<Dashboard />} />
+          <Route path="questions" element={<Questions />} />
+          <Route path="calendar" element={<Calendar />} />
+        </Route>
       </Routes>
     </div>
   )