feat(i18n): U3 国际化 — react-i18next 双语框架 + 侧边栏/Header/Login 全量翻译 + 语言切换器

- 安装 i18next + react-i18next，创建 src/i18n/ 配置实例
- 创建 zh-CN.json (120+ 键) + en-US.json 全量英文翻译
- 侧边栏 42+ 菜单项全量 labelKey 国际化
- Header 通知/主题/用户菜单全量翻译
- Login 页面表单/按钮/主题+语言切换器
- zustand useLanguageStore 持久化语言选择
- HTML lang 属性自动同步
- tsc -b=0, npm build=✅, 17/17 测试通过

frontend/src/i18n/index.ts

@@ -0,0 +1,52 @@
+import i18n from 'i18next';
+import { initReactI18next } from 'react-i18next';
+import zhCN from '@/locales/zh-CN.json';
+import enUS from '@/locales/en-US.json';
+
+export type AppLanguage = 'zh-CN' | 'en-US';
+
+export const SUPPORTED_LANGUAGES: { code: AppLanguage; label: string; flag: string }[] = [
+  { code: 'zh-CN', label: '中文', flag: '🇨🇳' },
+  { code: 'en-US', label: 'English', flag: '🇺🇸' },
+];
+
+export const DEFAULT_LANGUAGE: AppLanguage = 'zh-CN';
+
+/** Read persisted language from localStorage (zustand persist key) */
+function getPersistedLanguage(): AppLanguage {
+  try {
+    const raw = localStorage.getItem('language-storage');
+    if (raw) {
+      const parsed = JSON.parse(raw);
+      const lang = parsed?.state?.language;
+      if (lang === 'zh-CN' || lang === 'en-US') return lang;
+    }
+  } catch {
+    // ignore parse errors
+  }
+  return DEFAULT_LANGUAGE;
+}
+
+i18n.use(initReactI18next).init({
+  resources: {
+    'zh-CN': { translation: zhCN },
+    'en-US': { translation: enUS },
+  },
+  lng: getPersistedLanguage(),
+  fallbackLng: 'zh-CN',
+  interpolation: {
+    escapeValue: false, // React already escapes
+  },
+  react: {
+    useSuspense: false,
+  },
+});
+
+// Sync <html lang="..."> with current language
+i18n.on('languageChanged', (lng) => {
+  if (typeof document !== 'undefined') {
+    document.documentElement.lang = lng;
+  }
+});
+
+export default i18n;