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>

app/Http/Controllers/API/AuthController.php

@@ -474,7 +474,7 @@ class AuthController extends Controller
             'email' => 'required|string|email|max:255|unique:users',
             'password' => 'required|string|min:6|confirmed',
             'phone' => 'nullable|string|max:20',
-            'business_name' => 'required|string|max:255',
+            'business_name' => 'nullable|string|max:255',
             'description' => 'nullable|string',
             'contact_person' => 'nullable|string|max:100',
             'contact_phone' => 'nullable|string|max:20',
@@ -655,6 +655,12 @@ class AuthController extends Controller
             'contact_email' => 'nullable|string|email|max:255',
             'address' => 'nullable|string|max:255',
             'business_hours' => 'nullable|string|max:100',
+            'certifications' => 'nullable|string',
+            'dive_sites' => 'nullable|string',
+            'services' => 'nullable|string',
+            'facilities' => 'nullable|string',
+            'website' => 'nullable|string|max:255',
+            'social_media' => 'nullable|string|max:255',
         ]);
 
         if ($validator->fails()) {
@@ -701,7 +707,25 @@ class AuthController extends Controller
         if ($request->has('business_hours')) {
             $providerProfile->business_hours = $request->business_hours;
         }
-        
+        if ($request->has('certifications')) {
+            $providerProfile->certifications = $request->certifications;
+        }
+        if ($request->has('dive_sites')) {
+            $providerProfile->dive_sites = $request->dive_sites;
+        }
+        if ($request->has('services')) {
+            $providerProfile->services = $request->services;
+        }
+        if ($request->has('facilities')) {
+            $providerProfile->facilities = $request->facilities;
+        }
+        if ($request->has('website')) {
+            $providerProfile->website = $request->website;
+        }
+        if ($request->has('social_media')) {
+            $providerProfile->social_media = $request->social_media;
+        }
+
         $providerProfile->save();
 
         // 加載服務提供者資料