feat(viewer): vite vue3 scaffold with routing and i18n

- apps/viewer (@photo/viewer): Vite 5 + Vue 3.4 + TS 最小骨架（无 Pinia/UI 库）
- router: /a/:token 活动页 + /a/:token/p/:photoId 单图深链（复用活动页组件）
- i18n: vue-i18n 9，zh-CN/en 词条，localStorage(viewer.locale) 持久 +
  navigator.language 检测兜底 zh-CN；顶栏语言切换组件；文案全量走 i18n
- api client: 类型化 fetch 封装（契约生成类型直连），错误信封 → ApiError，
  错误码 → i18n key 映射表（src/api/errors.ts）
- scripts: dev/build/preview/test(vitest 10 例)/test:e2e(Playwright 冒烟,
  webServer 自拉 dev)/lint/vue-tsc 全部接入 turbo
- root: +@playwright/test、+eslint-plugin-vue（flat config 收敛到 apps/**/*.vue）、
  pnpm.onlyBuiltDependencies 放行 esbuild

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

apps/viewer/src/__tests__/i18n.spec.ts

@@ -0,0 +1,35 @@
+import { mount } from '@vue/test-utils';
+import { beforeEach, describe, expect, it } from 'vitest';
+import { nextTick } from 'vue';
+import App from '../App.vue';
+import { LOCALE_STORAGE_KEY, i18n } from '../i18n';
+import { router } from '../router';
+
+describe('viewer i18n', () => {
+  beforeEach(() => {
+    localStorage.clear();
+    i18n.global.locale.value = 'zh-CN';
+  });
+
+  it('switching locale re-renders messages and persists the choice', async () => {
+    await router.push('/a/tok-i18n');
+    await router.isReady();
+    const wrapper = mount(App, { global: { plugins: [router, i18n] } });
+
+    expect(wrapper.text()).toContain('照片即将上线');
+    expect(document.title).toBe('现场相册');
+
+    await wrapper.find('button.locale-switcher').trigger('click');
+    await nextTick();
+    expect(wrapper.text()).toContain('Photos coming soon');
+    expect(document.title).toBe('Live Gallery');
+    expect(document.documentElement.getAttribute('lang')).toBe('en');
+    expect(localStorage.getItem(LOCALE_STORAGE_KEY)).toBe('en');
+
+    // 切回 zh-CN 同样生效
+    await wrapper.find('button.locale-switcher').trigger('click');
+    await nextTick();
+    expect(wrapper.text()).toContain('照片即将上线');
+    expect(localStorage.getItem(LOCALE_STORAGE_KEY)).toBe('zh-CN');
+  });
+});