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/services/adminApi.ts

@@ -0,0 +1,140 @@
+const API_URL = import.meta.env.VITE_API_URL || 'http://localhost:8000'
+
+// Helper para obtener headers con auth
+const getAuthHeaders = (token: string | null) => ({
+  'Content-Type': 'application/json',
+  ...(token ? { 'Authorization': `Bearer ${token}` } : {})
+})
+
+// Auth
+export const adminLogin = async (username: string, password: string) => {
+  const formData = new FormData()
+  formData.append('username', username)
+  formData.append('password', password)
+
+  const response = await fetch(`${API_URL}/api/admin/login`, {
+    method: 'POST',
+    body: formData
+  })
+
+  if (!response.ok) {
+    const error = await response.json()
+    throw new Error(error.detail || 'Login failed')
+  }
+
+  return response.json()
+}
+
+export const adminRegister = async (username: string, password: string) => {
+  const response = await fetch(`${API_URL}/api/admin/register`, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ username, password })
+  })
+
+  if (!response.ok) {
+    const error = await response.json()
+    throw new Error(error.detail || 'Registration failed')
+  }
+
+  return response.json()
+}
+
+// Questions
+export const getQuestions = async (token: string, categoryId?: number, status?: string) => {
+  const params = new URLSearchParams()
+  if (categoryId) params.append('category_id', String(categoryId))
+  if (status) params.append('status', status)
+
+  const response = await fetch(`${API_URL}/api/admin/questions?${params}`, {
+    headers: getAuthHeaders(token)
+  })
+
+  if (!response.ok) throw new Error('Failed to fetch questions')
+  return response.json()
+}
+
+export const createQuestion = async (token: string, data: {
+  category_id: number
+  question_text: string
+  correct_answer: string
+  alt_answers?: string[]
+  difficulty: number
+  date_active?: string
+  fun_fact?: string
+}) => {
+  const response = await fetch(`${API_URL}/api/admin/questions`, {
+    method: 'POST',
+    headers: getAuthHeaders(token),
+    body: JSON.stringify(data)
+  })
+
+  if (!response.ok) throw new Error('Failed to create question')
+  return response.json()
+}
+
+export const updateQuestion = async (token: string, id: number, data: Record<string, unknown>) => {
+  const response = await fetch(`${API_URL}/api/admin/questions/${id}`, {
+    method: 'PUT',
+    headers: getAuthHeaders(token),
+    body: JSON.stringify(data)
+  })
+
+  if (!response.ok) throw new Error('Failed to update question')
+  return response.json()
+}
+
+export const deleteQuestion = async (token: string, id: number) => {
+  const response = await fetch(`${API_URL}/api/admin/questions/${id}`, {
+    method: 'DELETE',
+    headers: getAuthHeaders(token)
+  })
+
+  if (!response.ok) throw new Error('Failed to delete question')
+  return response.json()
+}
+
+export const generateQuestions = async (token: string, data: {
+  category_id: number
+  difficulty: number
+  count: number
+}) => {
+  const response = await fetch(`${API_URL}/api/admin/questions/generate`, {
+    method: 'POST',
+    headers: getAuthHeaders(token),
+    body: JSON.stringify(data)
+  })
+
+  if (!response.ok) throw new Error('Failed to generate questions')
+  return response.json()
+}
+
+export const approveQuestion = async (token: string, id: number) => {
+  const response = await fetch(`${API_URL}/api/admin/questions/${id}/approve`, {
+    method: 'POST',
+    headers: getAuthHeaders(token)
+  })
+
+  if (!response.ok) throw new Error('Failed to approve question')
+  return response.json()
+}
+
+export const rejectQuestion = async (token: string, id: number) => {
+  const response = await fetch(`${API_URL}/api/admin/questions/${id}/reject`, {
+    method: 'POST',
+    headers: getAuthHeaders(token)
+  })
+
+  if (!response.ok) throw new Error('Failed to reject question')
+  return response.json()
+}
+
+// Categories
+export const getCategories = async (token: string) => {
+  const response = await fetch(`${API_URL}/api/admin/categories`, {
+    headers: getAuthHeaders(token)
+  })
+
+  if (!response.ok) throw new Error('Failed to fetch categories')
+  return response.json()
+}