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-route.test.ts

@@ -1,9 +1,22 @@
 import type { IncomingMessage, ServerResponse } from 'node:http';
+import { mkdtemp, readFile, rm, writeFile } from 'node:fs/promises';
+import { tmpdir } from 'node:os';
+import { join } from 'node:path';
 import { Readable, Writable } from 'node:stream';
-import { describe, expect, it, vi } from 'vitest';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { handleImageWorkspaceRoutes } from '@electron/api/routes/image-workspace';
 import type { HostApiContext } from '@electron/api/context';
 
+const electronMocks = vi.hoisted(() => ({
+  getPath: vi.fn(),
+  showSaveDialog: vi.fn(),
+}));
+
+vi.mock('electron', () => ({
+  app: { getPath: electronMocks.getPath },
+  dialog: { showSaveDialog: electronMocks.showSaveDialog },
+}));
+
 function createResponse() {
   const chunks: string[] = [];
   const res = {
@@ -68,6 +81,21 @@ const bootstrap = {
 };
 
 describe('AI design Main route boundary', () => {
+  let temporaryDirectory: string | null = null;
+
+  beforeEach(() => {
+    electronMocks.getPath.mockReset();
+    electronMocks.getPath.mockReturnValue(tmpdir());
+    electronMocks.showSaveDialog.mockReset();
+  });
+
+  afterEach(async () => {
+    if (temporaryDirectory) {
+      await rm(temporaryDirectory, { recursive: true, force: true });
+      temporaryDirectory = null;
+    }
+  });
+
   it('does not claim unrelated routes', async () => {
     const response = createResponse();
     const handled = await handleImageWorkspaceRoutes(
@@ -107,6 +135,7 @@ describe('AI design Main route boundary', () => {
       submitMessage: vi.fn().mockResolvedValue({ workspaceId: 'workspace/one' }),
       confirmGeneration: vi.fn().mockResolvedValue({ workspaceId: 'workspace/one' }),
       listTasks: vi.fn().mockResolvedValue([]),
+      uploadAsset: vi.fn().mockResolvedValue({ assetId: 'asset-uploaded' }),
       openAssetContent: vi.fn(),
       getCapabilities: vi.fn(),
       reset: vi.fn().mockResolvedValue(bootstrap),
@@ -160,6 +189,30 @@ describe('AI design Main route boundary', () => {
       attachmentAssetIds: [],
     });
 
+    const uploadResponse = createResponse();
+    await handleImageWorkspaceRoutes(
+      createRequest('POST', {
+        fileName: 'poster.webp',
+        mimeType: 'image/webp',
+        dataBase64: 'AQID',
+      }),
+      uploadResponse.res,
+      new URL(
+        'http://127.0.0.1/api/works/image-workspace/workspaces/workspace%2Fone/assets',
+      ),
+      ctx,
+    );
+    expect(workspace.uploadAsset).toHaveBeenCalledWith({
+      workspaceId: 'workspace/one',
+      fileName: 'poster.webp',
+      mimeType: 'image/webp',
+      bytes: Buffer.from([1, 2, 3]),
+    });
+    expect(uploadResponse.json()).toMatchObject({
+      success: true,
+      data: { assetId: 'asset-uploaded' },
+    });
+
     const confirmResponse = createResponse();
     await handleImageWorkspaceRoutes(
       createRequest('POST', {
@@ -216,6 +269,116 @@ describe('AI design Main route boundary', () => {
     expect(Buffer.concat(response.chunks).toString()).toBe('partial');
   });
 
+  it('streams a private image asset into the native save location', async () => {
+    temporaryDirectory = await mkdtemp(join(tmpdir(), 'makelore-design-download-'));
+    const savedPath = join(temporaryDirectory, 'design.png');
+    electronMocks.showSaveDialog.mockResolvedValue({ canceled: false, filePath: savedPath });
+    const openAssetContent = vi.fn().mockResolvedValue(new Response('image-bytes', {
+      status: 200,
+      headers: { 'Content-Type': 'image/png' },
+    }));
+    const response = createResponse();
+
+    await handleImageWorkspaceRoutes(
+      createRequest('POST', { defaultFileName: 'Ocean poster.png' }),
+      response.res,
+      new URL(
+        'http://127.0.0.1/api/works/image-workspace/workspaces/workspace%2Fone/assets/asset%2Fone/download',
+      ),
+      { imageWorkspace: { openAssetContent } } as unknown as HostApiContext,
+    );
+
+    expect(electronMocks.showSaveDialog).toHaveBeenCalledWith(expect.objectContaining({
+      defaultPath: expect.stringContaining('Ocean poster.png'),
+    }));
+    expect(openAssetContent).toHaveBeenCalledWith('workspace/one', 'asset/one');
+    expect(response.json()).toMatchObject({
+      success: true,
+      data: { status: 'saved' },
+    });
+    await expect(readFile(savedPath, 'utf8')).resolves.toBe('image-bytes');
+  });
+
+  it('preserves an existing destination when the private image stream fails', async () => {
+    temporaryDirectory = await mkdtemp(join(tmpdir(), 'makelore-design-download-'));
+    const savedPath = join(temporaryDirectory, 'existing.png');
+    await writeFile(savedPath, 'original');
+    electronMocks.showSaveDialog.mockResolvedValue({ canceled: false, filePath: savedPath });
+    const body = new ReadableStream<Uint8Array>({
+      start(controller) {
+        controller.enqueue(new TextEncoder().encode('partial'));
+        controller.error(new Error('connection lost'));
+      },
+    });
+    const response = createResponse();
+
+    await handleImageWorkspaceRoutes(
+      createRequest('POST', { defaultFileName: 'Ocean poster.png' }),
+      response.res,
+      new URL(
+        'http://127.0.0.1/api/works/image-workspace/workspaces/workspace-one/assets/asset-one/download',
+      ),
+      {
+        imageWorkspace: {
+          openAssetContent: vi.fn().mockResolvedValue(new Response(body, {
+            status: 200,
+            headers: { 'Content-Type': 'image/png' },
+          })),
+        },
+      } as unknown as HostApiContext,
+    );
+
+    expect(response.json()).toMatchObject({
+      success: false,
+      code: 'DESIGN_ASSET_SAVE_FAILED',
+    });
+    await expect(readFile(savedPath, 'utf8')).resolves.toBe('original');
+  });
+
+  it('does not fetch private bytes when the native save dialog is cancelled', async () => {
+    electronMocks.showSaveDialog.mockResolvedValue({ canceled: true });
+    const openAssetContent = vi.fn();
+    const response = createResponse();
+
+    await handleImageWorkspaceRoutes(
+      createRequest('POST', { defaultFileName: 'Ocean poster.png' }),
+      response.res,
+      new URL(
+        'http://127.0.0.1/api/works/image-workspace/workspaces/workspace-one/assets/asset-one/download',
+      ),
+      { imageWorkspace: { openAssetContent } } as unknown as HostApiContext,
+    );
+
+    expect(openAssetContent).not.toHaveBeenCalled();
+    expect(response.json()).toMatchObject({
+      success: true,
+      data: { status: 'cancelled' },
+    });
+  });
+
+  it('rejects an oversized image upload before reading it into the Main process', async () => {
+    const uploadAsset = vi.fn();
+    const request = createRequest('POST', {});
+    request.headers['content-length'] = String(14 * 1024 * 1024);
+    const response = createResponse();
+
+    await handleImageWorkspaceRoutes(
+      request,
+      response.res,
+      new URL(
+        'http://127.0.0.1/api/works/image-workspace/workspaces/workspace-one/assets',
+      ),
+      { imageWorkspace: { uploadAsset } } as unknown as HostApiContext,
+    );
+
+    expect(response.statusCode).toBe(413);
+    expect(response.json()).toMatchObject({
+      success: false,
+      code: 'design_asset_upload_too_large',
+    });
+    expect(uploadAsset).not.toHaveBeenCalled();
+  });
+
   it('relays normalized task events over local SSE and forwards the opaque resume cursor', async () => {
     const close = vi.fn();
     const openWorkspaceEvents = vi.fn().mockResolvedValue({