feat(i18n): T15 full-stack language audit with coverage assertions

- viewer/admin 错误码→文案映射按契约 ErrorCodes 枚举全集登记（机器守护
  spec 遍历枚举断言无缺漏 + 映射键在 zh/en 词条均存在 + locale 键集
  递归相等）；mobile 镜像补齐 DOWNLOAD_DISABLED（P4a 增补漏登）
- 漏译修复：viewer.photoRemoved en 侧缺失（审计测试捕获）；
  DOWNLOAD_DISABLED 观众侧专属文案 + useDownload 专属提示接线；
  admin apiMsg 增 RATE_LIMITED/ALBUM_NOT_PUBLIC/DOWNLOAD_DISABLED/
  SYNC_AUTH_FAILED 四码词条
- 运行时抽查：admin 主旅程 en 版 e2e（登录→相册管理全英文 + 偏好
  持久）；embed iframe lang=en 渲染与深链跟随 e2e（zh 侧既有用例覆盖）
- contracts 新增 ./errors 浏览器安全子路径（errors.ts 纯 TS）；
  admin 增 @photo/contracts devDep（仅测试消费）

Co-Authored-By: Claude Code <noreply@anthropic.com>

apps/admin/src/locales/langs/en.json

@@ -792,6 +792,10 @@
     "PHOTO_NOT_PUBLIC": "Photo state cannot be hidden (not published)",
     "UPLOAD_VERIFY_FAILED": "Upload verification failed, please retry",
     "UPLOAD_SESSION_CONFLICT": "Upload session conflict, please start over",
-    "ALBUM_UPLOAD_DISABLED": "Upload is disabled for this album"
+    "ALBUM_UPLOAD_DISABLED": "Upload is disabled for this album",
+    "RATE_LIMITED": "Too many requests, please try again later",
+    "ALBUM_NOT_PUBLIC": "Album not found or offline",
+    "DOWNLOAD_DISABLED": "Downloads are not enabled for this album",
+    "SYNC_AUTH_FAILED": "Sync service authentication failed; check service configuration"
   }
 }

apps/admin/src/locales/langs/zh.json

@@ -792,6 +792,10 @@
     "PHOTO_NOT_PUBLIC": "照片状态不可隐藏（非已发布）",
     "UPLOAD_VERIFY_FAILED": "上传核验失败，请重试",
     "UPLOAD_SESSION_CONFLICT": "上传会话状态冲突，请重新开始",
-    "ALBUM_UPLOAD_DISABLED": "该相册已关闭上传"
+    "ALBUM_UPLOAD_DISABLED": "该相册已关闭上传",
+    "RATE_LIMITED": "操作过于频繁，请稍后再试",
+    "ALBUM_NOT_PUBLIC": "相册不存在或已下线",
+    "DOWNLOAD_DISABLED": "该相册未开放下载",
+    "SYNC_AUTH_FAILED": "同步服务鉴权失败，请检查服务配置"
   }
 }