feat(prompt): AI 提示词优化双栏交互

新增 prompt API 模块调用后端 /api/v1/prompt/optimize，PromptEditor
重构为左右双栏布局支持原始/优化提示词对比编辑，默认开启 AI 优化，
用户可关闭开关或手动编辑优化结果后再提交生成。

frontend/src/pages/GeneratePage.tsx

@@ -8,7 +8,7 @@ import ProgressBar from '../components/ProgressBar'
 export default function GeneratePage() {
   const { projectId = 'proj-default' } = useParams()
   const navigate = useNavigate()
-  const { prompt, assetType, reset: resetTask } = useTaskStore()
+  const { assetType, reset: resetTask } = useTaskStore()
   const {
     status,
     stage,
@@ -35,8 +35,8 @@ export default function GeneratePage() {
     }
   }, [status, taskId, projectId, navigate])
 
-  const handleSubmit = async () => {
-    await submit(projectId, prompt, assetType)
+  const handleSubmit = async (finalPrompt: string) => {
+    await submit(projectId, finalPrompt, assetType)
   }
 
   const handleReset = () => {