docs(design): DEAI-2 逐条处理情况与不做原因;补下载器原因码

Co-Authored-By: Claude Opus 5.5 <[email protected]>

packages/contracts/src/reasons.ts

@@ -38,6 +38,8 @@ const REASONS: Record<string, ReasonText> = {
   already_in_library: { text: "已在媒体库里" },
   main_not_transferred: { text: "主文件没有整理，附属文件一起跳过" },
   timeout: { text: "操作超时", action: "稍后重试" },
+  downloader_unavailable: { text: "下载器连不上", action: "在设置 → 下载器里测试连接" },
+  downloader_error: { text: "下载器报错", action: "查看下载器日志" },
 };
 
 /**