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/__tests__/download.spec.ts

@@ -2,6 +2,7 @@ import { flushPromises, mount, type VueWrapper } from '@vue/test-utils';
 import { defineComponent, ref } from 'vue';
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import {
+  ApiError,
   getPhotoDownload,
   getPublicPhotos,
   type PublicAlbum,
@@ -250,6 +251,20 @@ describe('photo download (P4a Task 4)', () => {
     expect(openSpy).not.toHaveBeenCalled();
   });
 
+  it('DOWNLOAD_DISABLED surfaces the dedicated localized notice (T15 漏译修复)', async () => {
+    // 相册中途关闭下载开关（载荷仍开但端点 403）→ 专属文案而非通用失败。
+    mockedGetPhotoDownload.mockReset().mockRejectedValue(
+      new ApiError('DOWNLOAD_DISABLED', 'downloads disabled', 403, null),
+    );
+    const wrapper = mountLightbox(makeAlbum());
+    await flushPromises();
+
+    await wrapper.find('.control.download').trigger('click');
+    await flushPromises();
+    expect(wrapper.find('.download-failed[role="alert"]').text()).toContain('该相册未开放下载');
+    expect(wrapper.find('.download-failed').text()).not.toContain('下载失败');
+  });
+
   it('WeChat webview: click opens the guidance overlay instead of hitting the endpoint', async () => {
     stubUserAgent(WECHAT_IOS_UA);
     const wrapper = mountLightbox(makeAlbum());