feat: 管线四阶段进度上报 + 图片预览 + Vite 代理

后端:
- pipeline.go: ProgressReporter 回调类型 + WithProgressReporter 注入 context
- nodes.go: 各节点 pre/post handler 调用 reportProgress() 上报阶段进度
- generate.go: TaskResponse 新增 stage 字段，runPipelineBg 注入进度回调

前端:
- vite.config.ts: 添加 /generation 代理到后端静态文件服务
- generation.ts: 轮询读取 stage 字段，暴露 stage/retryCount/rejectReason
- GeneratePage.tsx: ProgressBar 接收真实管线阶段数据

backend/internal/handler/generate.go

@@ -49,6 +49,7 @@ type TaskResponse struct {
 	Prompt     string `json:"prompt"`
 	AssetType  string `json:"assetType"`
 	Status     string `json:"status"`
+	Stage      string `json:"stage,omitempty"`
 	Progress   int    `json:"progress"`
 	RetryCount int    `json:"retryCount"`
 	Error      string `json:"error,omitempty"`
@@ -110,7 +111,12 @@ func Generate(c *gin.Context) {
 
 // runPipelineBg 后台执行生成管线，更新任务状态。
 func runPipelineBg(projectID, taskID string, req GenerateRequest) {
-	updateStatus(taskID, "running", 10)
+	// 注入进度上报回调
+	ctx := service.WithProgressReporter(context.Background(), func(stage string, progress int) {
+		updateTaskProgress(taskID, "running", stage, progress)
+	})
+
+	updateTaskProgress(taskID, "running", "prompt_builder", 5)
 
 	in := service.PipelineInput{
 		ProjectID:    projectID,
@@ -131,7 +137,6 @@ func runPipelineBg(projectID, taskID string, req GenerateRequest) {
 		},
 	}
 
-	ctx := context.Background()
 	output, err := service.RunPipeline(ctx, in)
 	if err != nil {
 		log.Printf("[generate] task %s failed: %v", taskID, err)
@@ -139,7 +144,7 @@ func runPipelineBg(projectID, taskID string, req GenerateRequest) {
 		return
 	}
 
-	updateStatus(taskID, "saving", 80)
+	updateTaskProgress(taskID, "saving", "format_adapter", 90)
 
 	// 保存图片到 ../generation/{projectId}/{taskId}/
 	genDir := filepath.Join("..", "generation", projectID, taskID)
@@ -180,13 +185,14 @@ func runPipelineBg(projectID, taskID string, req GenerateRequest) {
 	log.Printf("[generate] task %s completed, %d assets", taskID, len(assets))
 }
 
-func updateStatus(taskID, status string, progress int) {
+func updateTaskProgress(taskID, status, stage string, progress int) {
 	rec, ok := taskStore.Load(taskID)
 	if !ok {
 		return
 	}
 	r := rec.(*taskRecord)
 	r.task.Status = status
+	r.task.Stage = stage
 	r.task.Progress = progress
 	taskStore.Store(taskID, r)
 }