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/image-workspace-api.test.ts

@@ -13,7 +13,9 @@ import {
   ImageWorkspaceApiError,
   openImageWorkspaceTaskEvents,
   resolveImageWorkspaceAssetUrl,
+  saveImageWorkspaceAsset,
   sendImageWorkspaceMessage,
+  uploadImageWorkspaceAsset,
 } from '@/lib/image-workspace';
 
 vi.mock('@/lib/host-api', () => ({
@@ -99,6 +101,52 @@ describe('AI design renderer API boundary', () => {
     expect(String(init?.body)).not.toMatch(/model|resolution|outputCount/);
   });
 
+  it('sends the selected first-frame Asset id with the conversation turn', async () => {
+    await sendImageWorkspaceMessage(
+      'workspace-one',
+      5,
+      '使用作品图片作为视频首帧',
+      'turn-first-frame',
+      ['asset-first-frame'],
+    );
+
+    const [, init] = hostApiFetchMock.mock.calls[0];
+    expect(JSON.parse(String(init?.body))).toMatchObject({
+      clientTurnId: 'turn-first-frame',
+      expectedTurnRevision: 5,
+      attachmentAssetIds: ['asset-first-frame'],
+    });
+  });
+
+  it('uploads a supported local image through the authenticated Main boundary', async () => {
+    const asset = {
+      assetId: 'asset-uploaded',
+      workspaceId: 'workspace-one',
+      mediaType: 'image' as const,
+      mimeType: 'image/png',
+      width: 1200,
+      height: 800,
+      durationMilliseconds: null,
+      createdAt: '2026-08-06T10:00:00Z',
+      contentPath: '/api/works/image-workspace/workspaces/workspace-one/assets/asset-uploaded/content',
+    };
+    hostApiFetchMock.mockResolvedValueOnce({ success: true, status: 200, data: asset });
+    const file = new File([new Uint8Array([1, 2, 3])], 'poster.webp', {
+      type: 'image/webp',
+    });
+
+    await expect(uploadImageWorkspaceAsset('workspace-one', file)).resolves.toEqual(asset);
+
+    const [path, init] = hostApiFetchMock.mock.calls[0];
+    expect(path).toBe('/api/works/image-workspace/workspaces/workspace-one/assets');
+    expect(init).toMatchObject({ method: 'POST' });
+    expect(JSON.parse(String(init?.body))).toEqual({
+      fileName: 'poster.webp',
+      mimeType: 'image/webp',
+      dataBase64: 'AQID',
+    });
+  });
+
   it('confirms a Quote through a separate structured action route', async () => {
     await confirmImageWorkspaceGeneration('workspace-one', 5, 'quote/one');
 
@@ -132,4 +180,35 @@ describe('AI design renderer API boundary', () => {
         'http://127.0.0.1:13210/api/works/image-workspace/assets/one?token=host-token',
       );
   });
+
+  it('asks Main to save a private image asset without exposing its cloud URL', async () => {
+    hostApiFetchMock.mockResolvedValueOnce({
+      success: true,
+      status: 200,
+      data: { status: 'saved' },
+    });
+
+    await expect(saveImageWorkspaceAsset({
+      assetId: 'asset/one',
+      workspaceId: 'workspace/one',
+      mediaType: 'image',
+      mimeType: 'image/png',
+      width: 1024,
+      height: 1024,
+      durationMilliseconds: null,
+      createdAt: '2026-08-05T12:00:00Z',
+      contentPath: '/api/works/image-workspace/private/content',
+    })).resolves.toEqual({ status: 'saved' });
+
+    expect(hostApiFetchMock).toHaveBeenCalledWith(
+      '/api/works/image-workspace/workspaces/workspace%2Fone/assets/asset%2Fone/download',
+      {
+        method: 'POST',
+        body: JSON.stringify({
+          defaultFileName: 'Makelore-AI-Design-asset-one.png',
+        }),
+      },
+    );
+    expect(JSON.stringify(hostApiFetchMock.mock.calls[0])).not.toContain('private/content');
+  });
 });