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/viewer/src/i18n/locales/en.ts

@@ -51,6 +51,8 @@ export default {
     loadFailed: 'Failed to load image',
     // PHOTO_NOT_PUBLIC 专用：not found / unpublished / hidden share one code (anti-enumeration).
     photoUnavailable: 'Photo not found or unavailable',
+    // 轮播中被移除（ws photo.hidden）的轻提示。
+    photoRemoved: 'This photo has been removed',
     backToAlbum: 'Back to album',
   },
   route: { tokenLabel: 'Album token', photoLabel: 'Photo' },
@@ -67,5 +69,7 @@ export default {
     internal: 'Service temporarily unavailable, please try again later',
     unknown: 'Something went wrong, please try again later',
     network: 'Network error, please check your connection',
+    // DOWNLOAD_DISABLED (P4a two-tier download switches): visible but not downloadable.
+    downloadDisabled: 'Downloads are not enabled for this album',
   },
 };

apps/viewer/src/i18n/locales/zh-CN.ts

@@ -69,5 +69,7 @@ export default {
     internal: '服务暂时不可用，请稍后再试',
     unknown: '出错了，请稍后再试',
     network: '网络连接失败，请检查网络',
+    // DOWNLOAD_DISABLED（P4a 下载两级开关）：相册可见但未开放下载。
+    downloadDisabled: '该相册未开放下载',
   },
 };