feat: 添加登入頁面開發模式自動填入功能

- 登入頁面自動填入測試資料 (john@mail.com / 1qaz@WSX)
- 只在開發環境生效，生產環境保持原有行為
- 添加開發模式視覺提示
- 註冊頁面保持手動輸入確保流程完整性

提升開發效率，無需每次手動輸入登入資料

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/app/login/page.tsx

@@ -8,8 +8,12 @@ import { useAuth } from '@/contexts/AuthContext'
 export default function LoginPage() {
   const router = useRouter()
   const { login } = useAuth()
-  const [email, setEmail] = useState('')
-  const [password, setPassword] = useState('')
+  const [email, setEmail] = useState(
+    process.env.NODE_ENV === 'development' ? 'john@mail.com' : ''
+  )
+  const [password, setPassword] = useState(
+    process.env.NODE_ENV === 'development' ? '1qaz@WSX' : ''
+  )
   const [rememberMe, setRememberMe] = useState(false)
   const [loading, setLoading] = useState(false)
   const [error, setError] = useState('')
@@ -44,6 +48,13 @@ export default function LoginPage() {
         <div className="text-center mb-8">
           <h1 className="text-3xl font-bold text-gray-900">歡迎回來</h1>
           <p className="text-gray-600 mt-2">登入您的 DramaLing 帳號</p>
+          {process.env.NODE_ENV === 'development' && (
+            <div className="mt-4 p-2 bg-yellow-100 border border-yellow-300 rounded-lg">
+              <p className="text-yellow-800 text-xs">
+                🚧 開發模式 - 測試資料已自動填入
+              </p>
+            </div>
+          )}
         </div>
 
         <form onSubmit={handleSubmit} className="space-y-6">