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/App.vue

@@ -2,24 +2,29 @@
 import { computed, onMounted } from 'vue'
 import { useAuthStore } from './stores/auth'
 import { useCoachAuthStore } from './stores/coachAuth'
+import { useAdminAuthStore } from './stores/adminAuth'
 import { useRoute } from 'vue-router'
 import NavBar from './components/NavBar.vue'
 
 const auth      = useAuthStore()
 const coachAuth = useCoachAuthStore()
+const adminAuth = useAdminAuthStore()
 const route     = useRoute()
 
 onMounted(() => {
   auth.init()
   coachAuth.init()
+  adminAuth.init()
 })
 
-const isCoachPage = computed(() => route.path.startsWith('/coach'))
+const isBackofficePage = computed(() =>
+  route.path.startsWith('/coach') || route.path.startsWith('/admin')
+)
 </script>
 
 <template>
   <div class="min-h-screen bg-gray-50">
-    <NavBar v-if="!isCoachPage" />
+    <NavBar v-if="!isBackofficePage" />
     <RouterView />
   </div>
 </template>