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。

src/stores/image-workspace.ts

@@ -60,7 +60,7 @@ type ImageWorkspaceState = {
   refreshTasks: () => Promise<DesignGenerationTask[]>;
   connectTaskStream: () => void;
   disconnectTaskStream: () => void;
-  sendMessage: (message: string) => Promise<DesignWorkspace>;
+  sendMessage: (message: string, attachmentAssetIds?: string[]) => Promise<DesignWorkspace>;
   confirmGeneration: (quoteId: string) => Promise<DesignWorkspace>;
   reset: () => void;
 };
@@ -635,19 +635,27 @@ export const useImageWorkspaceStore = create<ImageWorkspaceState>((set, get) =>
 
     disconnectTaskStream: stopTaskStream,
 
-    sendMessage: async (message) => {
+    sendMessage: async (message, attachmentAssetIds = []) => {
       const workspace = get().workspace;
       const userText = message.trim();
       const clientTurnId = createImageWorkspaceTurnId();
       if (!workspace) throw new Error('请先选择设计项目');
       try {
         set({ pendingTurn: createPendingTurn(workspace, clientTurnId, userText), error: null });
-        const updated = await sendImageWorkspaceMessage(
-          workspace.workspaceId,
-          workspace.turnRevision,
-          userText,
-          clientTurnId,
-        );
+        const updated = attachmentAssetIds.length > 0
+          ? await sendImageWorkspaceMessage(
+              workspace.workspaceId,
+              workspace.turnRevision,
+              userText,
+              clientTurnId,
+              attachmentAssetIds,
+            )
+          : await sendImageWorkspaceMessage(
+              workspace.workspaceId,
+              workspace.turnRevision,
+              userText,
+              clientTurnId,
+            );
         if (get().activeWorkspaceId === workspace.workspaceId) {
           applyWorkspace(updated);
           await get().refreshTasks().catch(() => []);