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/lib/image-workspace.ts

@@ -9,6 +9,9 @@ import {
   IMAGE_WORKSPACE_API_PATH,
   IMAGE_WORKSPACE_UNAVAILABLE_CODE,
   IMAGE_WORKSPACE_UNAVAILABLE_MESSAGE,
+  designAssetDownloadPath,
+  type DesignAsset,
+  type DesignAssetSaveResult,
   type DesignGenerationTask,
   type DesignWorkspace,
   type DesignWorkspaceBootstrap,
@@ -125,6 +128,7 @@ export function sendImageWorkspaceMessage(
   expectedTurnRevision: number,
   message: string,
   clientTurnId = createImageWorkspaceTurnId(),
+  attachmentAssetIds: string[] = [],
 ): Promise<DesignWorkspace> {
   return requestData(
     `${IMAGE_WORKSPACE_API_PATH}/workspaces/${encodeURIComponent(workspaceId)}/messages`,
@@ -134,7 +138,7 @@ export function sendImageWorkspaceMessage(
         clientTurnId,
         expectedTurnRevision,
         message: message.trim(),
-        attachmentAssetIds: [],
+        attachmentAssetIds,
       }),
     },
   );
@@ -178,3 +182,82 @@ export async function resolveImageWorkspaceAssetUrl(contentPath: string): Promis
   const separator = contentPath.includes('?') ? '&' : '?';
   return `${getHostApiBase()}${contentPath}${separator}token=${encodeURIComponent(token)}`;
 }
+
+const IMAGE_FILE_EXTENSIONS: Record<string, string> = {
+  'image/gif': 'gif',
+  'image/jpeg': 'jpg',
+  'image/png': 'png',
+  'image/svg+xml': 'svg',
+  'image/webp': 'webp',
+};
+
+function assetDownloadFileName(asset: DesignAsset): string {
+  const safeAssetId = asset.assetId
+    .replace(/[^a-zA-Z0-9_-]+/g, '-')
+    .replace(/^-+|-+$/g, '')
+    .slice(0, 32) || 'image';
+  const extension = IMAGE_FILE_EXTENSIONS[asset.mimeType.toLowerCase()] ?? 'png';
+  return `Makelore-AI-Design-${safeAssetId}.${extension}`;
+}
+
+export function saveImageWorkspaceAsset(asset: DesignAsset): Promise<DesignAssetSaveResult> {
+  return requestData(
+    designAssetDownloadPath(asset.workspaceId, asset.assetId),
+    {
+      method: 'POST',
+      body: JSON.stringify({ defaultFileName: assetDownloadFileName(asset) }),
+    },
+  );
+}
+
+const DESIGN_IMAGE_UPLOAD_MIME_TYPES = new Set([
+  'image/jpeg',
+  'image/png',
+  'image/webp',
+]);
+const MAX_DESIGN_IMAGE_UPLOAD_BYTES = 10 * 1024 * 1024;
+
+async function fileToBase64(file: File): Promise<string> {
+  const bytes = new Uint8Array(await file.arrayBuffer());
+  let encoded = '';
+  for (let offset = 0; offset < bytes.length; offset += 48 * 1024) {
+    const chunk = bytes.subarray(offset, offset + 48 * 1024);
+    let binary = '';
+    for (let index = 0; index < chunk.length; index += 0x8000) {
+      binary += String.fromCharCode(...chunk.subarray(index, index + 0x8000));
+    }
+    encoded += btoa(binary);
+  }
+  return encoded;
+}
+
+export async function uploadImageWorkspaceAsset(
+  workspaceId: string,
+  file: File,
+): Promise<DesignAsset> {
+  if (!DESIGN_IMAGE_UPLOAD_MIME_TYPES.has(file.type)) {
+    throw new ImageWorkspaceApiError(
+      400,
+      'design_asset_upload_type_invalid',
+      '请选择 JPEG、PNG 或 WebP 图片',
+    );
+  }
+  if (file.size <= 0 || file.size > MAX_DESIGN_IMAGE_UPLOAD_BYTES) {
+    throw new ImageWorkspaceApiError(
+      413,
+      'design_asset_upload_too_large',
+      '图片大小需在 10 MB 以内',
+    );
+  }
+  return requestData(
+    `${IMAGE_WORKSPACE_API_PATH}/workspaces/${encodeURIComponent(workspaceId)}/assets`,
+    {
+      method: 'POST',
+      body: JSON.stringify({
+        fileName: file.name,
+        mimeType: file.type,
+        dataBase64: await fileToBase64(file),
+      }),
+    },
+  );
+}