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/stores/coachAuth.js

@@ -0,0 +1,38 @@
+import { defineStore } from 'pinia'
+import { ref, computed } from 'vue'
+import coachApi from '../api/coachAxios'
+
+export const useCoachAuthStore = defineStore('coachAuth', () => {
+  const user  = ref(null)
+  const token = ref(null)
+
+  const isLoggedIn = computed(() => !!token.value)
+
+  function init() {
+    const savedToken = localStorage.getItem('coach_token')
+    const savedUser  = localStorage.getItem('coach_user')
+    if (savedToken) {
+      token.value = savedToken
+      user.value  = savedUser ? JSON.parse(savedUser) : null
+    }
+  }
+
+  function setAuth(userData, tokenValue) {
+    user.value  = userData
+    token.value = tokenValue
+    localStorage.setItem('coach_token', tokenValue)
+    localStorage.setItem('coach_user', JSON.stringify(userData))
+  }
+
+  async function logout() {
+    try {
+      await coachApi.post('/provider/logout')
+    } catch {}
+    user.value  = null
+    token.value = null
+    localStorage.removeItem('coach_token')
+    localStorage.removeItem('coach_user')
+  }
+
+  return { user, token, isLoggedIn, init, setAuth, logout }
+})