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/package.json

@@ -51,6 +51,7 @@
     "cmdk": "^1.1.1",
     "date-fns": "^4.1.0",
     "embla-carousel-react": "^8.6.0",
+    "i18next": "^26.3.3",
     "input-otp": "^1.4.2",
     "lucide-react": "^0.562.0",
     "next-themes": "^0.4.6",
@@ -58,6 +59,7 @@
     "react-day-picker": "^9.13.0",
     "react-dom": "^19.2.0",
     "react-hook-form": "^7.70.0",
+    "react-i18next": "^17.0.8",
     "react-resizable-panels": "^4.2.2",
     "react-router-dom": "^7.13.2",
     "recharts": "^2.15.4",