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/service/nodes.go

@@ -57,8 +57,10 @@ var promptOptimizerNode = compose.InvokableLambda(func(ctx context.Context, in P
 func promptOptimizerPreHandler(ctx context.Context, in PipelineInput, state *PipelineState) (PipelineInput, error) {
 	if state.RetryCount == 0 {
 		state.Input = in
+		reportProgress(ctx, "prompt_builder", 10)
 	} else if state.RejectReason != "" {
 		in.RejectReason = state.RejectReason
+		reportProgress(ctx, "prompt_builder", 30+state.RetryCount*10)
 	}
 	return in, nil
 }
@@ -66,6 +68,7 @@ func promptOptimizerPreHandler(ctx context.Context, in PipelineInput, state *Pip
 // promptOptimizerPostHandler 将最终提示词写入全局状态。
 func promptOptimizerPostHandler(ctx context.Context, out string, state *PipelineState) (string, error) {
 	state.FinalPrompt = out
+	reportProgress(ctx, "asset_generator", 35)
 	return out, nil
 }
 
@@ -82,6 +85,7 @@ var assetGeneratorNode = compose.InvokableLambda(func(ctx context.Context, promp
 // assetGeneratorPostHandler 将原始图片写入全局状态。
 func assetGeneratorPostHandler(ctx context.Context, out []GeneratedImage, state *PipelineState) ([]GeneratedImage, error) {
 	state.RawImages = out
+	reportProgress(ctx, "quality_supervisor", 60)
 	return out, nil
 }
 
@@ -104,11 +108,14 @@ var qualitySupervisorNode = compose.InvokableLambda(func(ctx context.Context, im
 
 		if pass {
 			state.NextNode = nodeFormatAdapter
+			reportProgress(ctx, "format_adapter", 85)
 		} else if state.RetryCount >= 3 {
 			state.NextNode = nodeFormatAdapter
+			reportProgress(ctx, "format_adapter", 85)
 		} else {
 			state.RetryCount++
 			state.NextNode = nodePromptOptimizer
+			reportProgress(ctx, "quality_supervisor", 50+state.RetryCount*10)
 		}
 
 		input = state.Input