feat: 结果页添加帧预览、GIF动画和导出选项

- 后端: GetAssets 返回素材级别 metadata，区分 frame/preview/spritesheet 类型
- 后端: 上传时区分素材类型标签，GIF URL 独立追踪
- 前端: 结果详情页默认展示拆分后的单帧网格
- 前端: 添加动作帧 GIF 预览弹窗
- 前端: 添加导出下拉菜单（整张精灵表/自动切分帧）
- 前端: 失败任务展示错误信息和重试次数

backend/internal/handler/generate.go

@@ -47,8 +47,8 @@ type GenerateResponse struct {
 
 // AssetsResponse 素材列表响应。
 type AssetsResponse struct {
-	Assets   []model.AssetResponse  `json:"assets"`
-	Metadata service.AssetMetadata  `json:"metadata"`
+	Assets   []model.AssetResponse `json:"assets"`
+	Metadata service.AssetMetadata `json:"metadata"`
 }
 
 // Generate 素材生成接口（异步）。
@@ -136,7 +136,7 @@ func runPipelineBg(ctx context.Context, projectID, taskID string, req GenerateRe
 	updateTaskInDB(ctx, taskID, "saving", "format_adapter", "", 90)
 
 	// 上传素材并保存到数据库
-	var lastCDNURL string
+	var lastGIFCDNURL string
 	for i, a := range output.Assets {
 		key := fmt.Sprintf("generation/%s/%s/%d.%s", projectID, taskID, i, a.Format)
 		cdnURL, err := storageSvc.Upload(ctx, key, a.Data)
@@ -145,13 +145,15 @@ func runPipelineBg(ctx context.Context, projectID, taskID string, req GenerateRe
 			updateTaskInDB(ctx, taskID, "failed", "", "上传素材失败: "+err.Error(), 0)
 			return
 		}
-		lastCDNURL = cdnURL
 
 		var assetMeta map[string]interface{}
 		if a.Format == "gif" {
 			assetMeta = map[string]interface{}{"index": i, "type": "preview"}
+			lastGIFCDNURL = cdnURL
+		} else if strings.Contains(a.URL, "spritesheet") {
+			assetMeta = map[string]interface{}{"index": i, "type": "spritesheet"}
 		} else {
-			assetMeta = map[string]interface{}{"index": i}
+			assetMeta = map[string]interface{}{"index": i, "type": "frame"}
 		}
 		metadataJSON, _ := json.Marshal(assetMeta)
 
@@ -169,8 +171,8 @@ func runPipelineBg(ctx context.Context, projectID, taskID string, req GenerateRe
 	}
 
 	// GIF URL 替换为实际上传后的 CDN 地址
-	if output.Metadata.GIFURL != "" && lastCDNURL != "" {
-		output.Metadata.GIFURL = lastCDNURL
+	if output.Metadata.GIFURL != "" && lastGIFCDNURL != "" {
+		output.Metadata.GIFURL = lastGIFCDNURL
 	}
 
 	var fullMetadata string
@@ -232,9 +234,10 @@ func GetAssets(c *gin.Context) {
 	response := []model.AssetResponse{}
 	for _, a := range assets {
 		response = append(response, model.AssetResponse{
-			Key:    a.Key,
-			URL:    storageSvc.GetSignedURL(a.Key),
-			Format: a.Format,
+			Key:      a.Key,
+			URL:      storageSvc.GetSignedURL(a.Key),
+			Format:   a.Format,
+			Metadata: a.Metadata,
 		})
 	}