feat:實作 Admin Panel — 平台管理後台

後端：
- AdminStatsController：總會員/教練/課程數統計 API
- AdminUserController：會員與教練列表、詳情、啟用/停用、教練驗證（toggle 反轉語意）
- AdminOfferController：全平台課程列表與刪除
- routes/api.php：新增 /api/admin/stats、members、providers、offers 等路由

前端（frontend/）：
- adminAuth store、adminAxios（第三套獨立認證）
- /admin/* 路由群組 + requiresAdmin guard
- AdminNavBar、AdminLayout
- App.vue：isCoachPage → isBackofficePage（/coach/* 和 /admin/* 皆隱藏會員 NavBar）
- LoginView、DashboardView（統計卡片）
- MembersView、ProvidersView（含驗證操作）、OffersView（含刪除確認）

OpenSpec：
- 新增 specs：admin-auth / admin-user-management / admin-offer-management / admin-stats / admin-panel-ui
- 歸檔：openspec/changes/archive/2026-05-10-admin-panel

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

frontend/src/router/index.js

@@ -1,6 +1,7 @@
 import { createRouter, createWebHistory } from 'vue-router'
 import { useAuthStore } from '../stores/auth'
 import { useCoachAuthStore } from '../stores/coachAuth'
+import { useAdminAuthStore } from '../stores/adminAuth'
 
 const routes = [
   // Member
@@ -15,7 +16,7 @@ const routes = [
   // Coach (public)
   { path: '/coach/login',    component: () => import('../views/coach/LoginView.vue') },
   { path: '/coach/register', component: () => import('../views/coach/RegisterView.vue') },
-  // Coach (protected) — wrapped in CoachLayout
+  // Coach (protected)
   {
     path: '/coach',
     component: () => import('../layouts/CoachLayout.vue'),
@@ -27,6 +28,21 @@ const routes = [
       { path: 'profile',         component: () => import('../views/coach/ProfileView.vue') },
     ],
   },
+
+  // Admin (public)
+  { path: '/admin/login', component: () => import('../views/admin/LoginView.vue') },
+  // Admin (protected)
+  {
+    path: '/admin',
+    component: () => import('../layouts/AdminLayout.vue'),
+    meta: { requiresAdmin: true },
+    children: [
+      { path: 'dashboard', component: () => import('../views/admin/DashboardView.vue') },
+      { path: 'members',   component: () => import('../views/admin/MembersView.vue') },
+      { path: 'providers', component: () => import('../views/admin/ProvidersView.vue') },
+      { path: 'offers',    component: () => import('../views/admin/OffersView.vue') },
+    ],
+  },
 ]
 
 const router = createRouter({
@@ -37,13 +53,11 @@ const router = createRouter({
 router.beforeEach((to) => {
   const auth      = useAuthStore()
   const coachAuth = useCoachAuthStore()
+  const adminAuth = useAdminAuthStore()
 
-  if (to.meta.requiresAuth && !auth.isLoggedIn) {
-    return { path: '/login' }
-  }
-  if (to.meta.requiresCoach && !coachAuth.isLoggedIn) {
-    return { path: '/coach/login' }
-  }
+  if (to.meta.requiresAuth  && !auth.isLoggedIn)      return { path: '/login' }
+  if (to.meta.requiresCoach && !coachAuth.isLoggedIn) return { path: '/coach/login' }
+  if (to.meta.requiresAdmin && !adminAuth.isLoggedIn) return { path: '/admin/login' }
 })
 
 export default router