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。

tests/unit/works-square-design-workspace.test.ts

@@ -223,6 +223,46 @@ describe('Works Square AI design adapter', () => {
     );
   });
 
+  it('uploads a local reference image as multipart data and maps the returned Asset', async () => {
+    const fetchMock = vi.fn<typeof fetch>().mockResolvedValueOnce(jsonResponse({
+      asset_id: 'asset-uploaded',
+      media_type: 'image',
+      mime_type: 'image/png',
+      width: 1200,
+      height: 800,
+      duration_milliseconds: null,
+      created_at: '2026-08-06T10:00:00Z',
+    }));
+    const adapter = new WorksSquareDesignWorkspace({
+      apiBaseUrl: 'https://square.example',
+      fetchImpl: fetchMock,
+    });
+
+    await expect(adapter.uploadAsset({
+      workspaceId: 'workspace-one',
+      fileName: 'poster.webp',
+      mimeType: 'image/webp',
+      bytes: new Uint8Array([1, 2, 3]),
+    })).resolves.toMatchObject({
+      assetId: 'asset-uploaded',
+      workspaceId: 'workspace-one',
+      mimeType: 'image/png',
+      contentPath: '/api/works/image-workspace/workspaces/workspace-one/assets/asset-uploaded/content',
+    });
+
+    const [url, init] = fetchMock.mock.calls[0];
+    expect(url).toBe('https://square.example/api/design/workspaces/workspace-one/assets');
+    expect(init?.method).toBe('POST');
+    expect((init?.headers as Record<string, string>).Authorization).toBe('Bearer access-one');
+    expect((init?.headers as Record<string, string>)['Content-Type']).toBeUndefined();
+    const form = init?.body as FormData;
+    expect(form.get('file')).toBeInstanceOf(File);
+    expect((form.get('file') as File).name).toBe('poster.webp');
+    expect(await (form.get('file') as File).arrayBuffer()).toEqual(
+      Uint8Array.from([1, 2, 3]).buffer,
+    );
+  });
+
   it('completes a design turn from the connected WebSocket without polling the run endpoint', async () => {
     const { sockets, webSocketFactory } = scriptedSockets([{ open: true }]);
     const fetchMock = vi.fn<typeof fetch>(async (input) => {