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

frontend/src/pages/GeneratePage.tsx

@@ -19,9 +19,12 @@ export default function GeneratePage() {
   const { style: projectStyle, loadProject } = useProjectStore()
   const {
     status,
+    stage,
     progress,
     taskId,
     statusText,
+    retryCount,
+    rejectReason,
     submit,
     reset: resetGeneration,
   } = useGenerationStore()
@@ -83,11 +86,11 @@ export default function GeneratePage() {
       ) : (
         <div style={{ display: 'flex', flexDirection: 'column', gap: 24 }}>
           <ProgressBar
-            stage={null}
+            stage={stage}
             progress={progress}
             status={status}
-            retryCount={0}
-            rejectReason={null}
+            retryCount={retryCount}
+            rejectReason={rejectReason}
           />
 
           {status === 'running' && (