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/views/coach/LoginView.vue

@@ -0,0 +1,76 @@
+<script setup>
+import { ref } from 'vue'
+import { useRouter, useRoute } from 'vue-router'
+import coachApi from '../../api/coachAxios'
+import { useCoachAuthStore } from '../../stores/coachAuth'
+
+const router    = useRouter()
+const route     = useRoute()
+const coachAuth = useCoachAuthStore()
+
+const email    = ref('')
+const password = ref('')
+const error    = ref('')
+const loading  = ref(false)
+
+const registeredMsg = route.query.registered ? '註冊成功，請登入。' : ''
+
+async function submit() {
+  error.value   = ''
+  loading.value = true
+  try {
+    const res  = await coachApi.post('/provider/login', {
+      email:    email.value,
+      password: password.value,
+    })
+    const { user, token } = res.data.data
+    coachAuth.setAuth(user, token)
+    router.push('/coach/dashboard')
+  } catch (e) {
+    error.value = e.response?.data?.message || '帳號或密碼錯誤'
+  } finally {
+    loading.value = false
+  }
+}
+</script>
+
+<template>
+  <main class="min-h-screen bg-gray-50 flex items-center justify-center px-4">
+    <div class="bg-white rounded-2xl shadow-lg w-full max-w-md p-8">
+
+      <div class="text-center mb-8">
+        <p class="text-gray-500 text-sm mb-1">CFDive 教練後台</p>
+        <h1 class="text-2xl font-bold text-gray-800">教練登入</h1>
+      </div>
+
+      <div v-if="registeredMsg" class="bg-green-50 text-green-700 text-sm rounded-lg px-4 py-3 mb-4">
+        {{ registeredMsg }}
+      </div>
+      <div v-if="error" class="bg-red-50 text-red-600 text-sm rounded-lg px-4 py-3 mb-4">
+        {{ error }}
+      </div>
+
+      <form @submit.prevent="submit" class="flex flex-col gap-4">
+        <div>
+          <label class="block text-sm text-gray-600 mb-1">Email</label>
+          <input v-model="email" type="email" required
+            class="w-full border border-gray-300 rounded-lg px-4 py-2 focus:outline-none focus:ring-2 focus:ring-gray-400" />
+        </div>
+        <div>
+          <label class="block text-sm text-gray-600 mb-1">密碼</label>
+          <input v-model="password" type="password" required
+            class="w-full border border-gray-300 rounded-lg px-4 py-2 focus:outline-none focus:ring-2 focus:ring-gray-400" />
+        </div>
+        <button type="submit" :disabled="loading"
+          class="bg-gray-900 hover:bg-gray-700 text-white font-semibold py-2.5 rounded-lg transition disabled:opacity-60">
+          {{ loading ? '登入中...' : '登入' }}
+        </button>
+      </form>
+
+      <p class="text-center text-sm text-gray-500 mt-6">
+        還沒有帳號？
+        <RouterLink to="/coach/register" class="text-gray-700 hover:underline font-medium">申請教練帳號</RouterLink>
+      </p>
+    </div>
+  </main>
+</template>