完善 AI 设计资产预览与视频首帧选择

需求：生成图片需要支持大图查看和本地下载；制作视频时需要从当前项目作品选择首帧，或上传本地图片。

实现：新增首帧选择弹窗、JPEG/PNG/WebP 有界上传、附件 Asset ID 透传、Main 到服务端 multipart 转发，并保留上传失败重试与私有图片下载链路。

验证：相关 57 项单测、TypeScript 类型检查、目标 ESLint 和 Vite 生产构建全部通过。

shared/image-workspace.ts

@@ -70,6 +70,19 @@ export type DesignAsset = {
   contentPath: string;
 };
 
+export type DesignAssetSaveResult = {
+  status: 'saved' | 'cancelled';
+};
+
+export type DesignImageUploadMimeType = 'image/jpeg' | 'image/png' | 'image/webp';
+
+export type DesignAssetUploadInput = {
+  workspaceId: string;
+  fileName: string;
+  mimeType: DesignImageUploadMimeType;
+  bytes: Uint8Array;
+};
+
 export type DesignGenerationTask = {
   taskId: string;
   workspaceId: string;
@@ -150,3 +163,7 @@ export type DesignConfirmGenerationInput = {
 export function designAssetContentPath(workspaceId: string, assetId: string): string {
   return `${IMAGE_WORKSPACE_API_PATH}/workspaces/${encodeURIComponent(workspaceId)}/assets/${encodeURIComponent(assetId)}/content`;
 }
+
+export function designAssetDownloadPath(workspaceId: string, assetId: string): string {
+  return `${IMAGE_WORKSPACE_API_PATH}/workspaces/${encodeURIComponent(workspaceId)}/assets/${encodeURIComponent(assetId)}/download`;
+}