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。

electron/image-workspace/works-square-workspace.ts

@@ -1,5 +1,6 @@
 import type {
   DesignAsset,
+  DesignAssetUploadInput,
   DesignAssistantDeltaEvent,
   DesignBrief,
   DesignCapabilities,
@@ -266,22 +267,26 @@ function mapTask(task: ServerTask): DesignGenerationTask {
     quoteId: task.quote_id,
     quotedDesignPoints: task.quoted_design_points,
     failureCode: task.failure_code,
-    resultAssets: task.result_assets.map((asset) => ({
-      assetId: asset.asset_id,
-      workspaceId: task.workspace_id,
-      mediaType: asset.media_type,
-      mimeType: asset.mime_type,
-      width: asset.width,
-      height: asset.height,
-      durationMilliseconds: asset.duration_milliseconds,
-      createdAt: asset.created_at,
-      contentPath: designAssetContentPath(task.workspace_id, asset.asset_id),
-    })),
+    resultAssets: task.result_assets.map((asset) => mapAsset(task.workspace_id, asset)),
     createdAt: task.created_at,
     updatedAt: task.updated_at,
   };
 }
 
+function mapAsset(workspaceId: string, asset: ServerAsset): DesignAsset {
+  return {
+    assetId: asset.asset_id,
+    workspaceId,
+    mediaType: asset.media_type,
+    mimeType: asset.mime_type,
+    width: asset.width,
+    height: asset.height,
+    durationMilliseconds: asset.duration_milliseconds,
+    createdAt: asset.created_at,
+    contentPath: designAssetContentPath(workspaceId, asset.asset_id),
+  };
+}
+
 function createClientId(prefix: string): string {
   const id = globalThis.crypto?.randomUUID?.()
     ?? `${Date.now()}-${Math.random().toString(16).slice(2)}`;
@@ -913,6 +918,20 @@ export class WorksSquareDesignWorkspace implements DesignWorkspaceModule {
     return tasks.map(mapTask);
   }
 
+  async uploadAsset(input: DesignAssetUploadInput): Promise<DesignAsset> {
+    const form = new FormData();
+    form.set(
+      'file',
+      new Blob([Uint8Array.from(input.bytes)], { type: input.mimeType }),
+      input.fileName,
+    );
+    const asset = await this.requestJson<ServerAsset>(
+      `/api/design/workspaces/${encodeURIComponent(input.workspaceId)}/assets`,
+      { method: 'POST', body: form },
+    );
+    return mapAsset(input.workspaceId, asset);
+  }
+
   async openWorkspaceEvents(
     input: DesignWorkspaceEventSubscriptionInput,
   ): Promise<DesignWorkspaceEventSubscription> {
@@ -1141,7 +1160,9 @@ export class WorksSquareDesignWorkspace implements DesignWorkspaceModule {
       ...init,
       headers: {
         Accept: 'application/json',
-        ...(init.body ? { 'Content-Type': 'application/json' } : {}),
+        ...(init.body && !(init.body instanceof FormData)
+          ? { 'Content-Type': 'application/json' }
+          : {}),
         ...(init.headers ?? {}),
       },
     });