feat:實作 Coach Portal — 教練後台課程管理

後端：
- Migration：diving_offers 新增 provider_id（nullable FK）
- Migration：users.role ENUM 加入 provider 值
- Migration：diving_offers.spot 改為 nullable
- AuthController：registerProvider business_name 改為選填
- AuthController：updateProviderProfile 補上 certifications / dive_sites / services / facilities / website / social_media
- ProviderOfferController：教練課程 CRUD（index/show/store/update/destroy），實作 provider_id 所有權不變式（404 → 403 兩步驟）

前端（frontend/）：
- coachAuth store、coachAxios（獨立於 member auth）
- /coach/* 路由群組 + beforeEach guard
- CoachNavBar、CoachLayout（教練頁隱藏會員 NavBar）
- LoginView、RegisterView、DashboardView（表格 + 刪除確認）
- OfferFormView（新增/編輯共用）、ProfileView

OpenSpec：
- openspec/config.yaml 補入專案 context 與 rules
- 新增 specs：coach-offers-api / coach-portal-ui / provider-auth
- 更新 spec：diving-offers-api 加入 provider_id
- 歸檔：openspec/changes/archive/2026-05-10-coach-portal

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

frontend/src/App.vue

@@ -1,15 +1,25 @@
 <script setup>
-import { onMounted } from 'vue'
+import { computed, onMounted } from 'vue'
 import { useAuthStore } from './stores/auth'
+import { useCoachAuthStore } from './stores/coachAuth'
+import { useRoute } from 'vue-router'
 import NavBar from './components/NavBar.vue'
 
-const auth = useAuthStore()
-onMounted(() => auth.init())
+const auth      = useAuthStore()
+const coachAuth = useCoachAuthStore()
+const route     = useRoute()
+
+onMounted(() => {
+  auth.init()
+  coachAuth.init()
+})
+
+const isCoachPage = computed(() => route.path.startsWith('/coach'))
 </script>
 
 <template>
   <div class="min-h-screen bg-gray-50">
-    <NavBar />
+    <NavBar v-if="!isCoachPage" />
     <RouterView />
   </div>
 </template>