perf(frontend): U48 i18n locale 懒加载 — en-US JSON 动态导入拆分为独立 chunk

问题: src/i18n/index.ts 静态导入 zh-CN.json (200KB compact) + en-US.json
(292KB compact)，两者共 486KB 全部内联到主 index chunk，致主 chunk 696KB
超过 Vite 600KB 警告阈值。

修复:
- src/i18n/index.ts: 移除 en-US.json 静态导入，改为 ensureLocaleLoaded()
  动态导入 + i18n.addResourceBundle() 按需加载
- zh-CN (默认/回退语言) 保持静态内联，无行为变化
- en-US 仅在用户切换英文时按需 fetch (290KB 独立 chunk)
- 已选英文的用户刷新页面时异步加载 en-US (短暂中文闪烁，可接受)
- src/store/index.ts: setLanguage 调用 ensureLocaleLoaded 确保目标语言包
  加载完成后再切换

结果:
- 主 index chunk: 696KB → 423KB (-39%, -273KB)
- en-US chunk: 290KB 独立懒加载 (仅英文用户首次切换时 fetch)
- Vite 600KB chunk 警告消除
- tsc -b=0, npm build✓ (18.18s), ESLint=0/43/any=0, go build=0
- 17/17 集成测试通过, Docker healthy

frontend/src/i18n/index.ts

@@ -1,7 +1,6 @@
 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';
 
@@ -12,6 +11,24 @@ export const SUPPORTED_LANGUAGES: { code: AppLanguage; label: string; flag: stri
 
 export const DEFAULT_LANGUAGE: AppLanguage = 'zh-CN';
 
+/** Track which locale resource bundles have been registered with i18next. */
+const loadedLocales = new Set<string>(['zh-CN']);
+
+/**
+ * Ensure a locale resource bundle is loaded before use.
+ * zh-CN is bundled inline (default/fallback language).
+ * en-US is lazy-loaded via dynamic import to reduce the main bundle by ~290 kB.
+ * Subsequent calls for an already-loaded locale are instant no-ops.
+ */
+export async function ensureLocaleLoaded(lang: string): Promise<void> {
+  if (loadedLocales.has(lang)) return;
+  if (lang === 'en-US') {
+    const mod = await import('@/locales/en-US.json');
+    i18n.addResourceBundle('en-US', 'translation', mod.default, true, true);
+    loadedLocales.add('en-US');
+  }
+}
+
 /** Read persisted language from localStorage (zustand persist key) */
 function getPersistedLanguage(): AppLanguage {
   try {
@@ -27,12 +44,15 @@ function getPersistedLanguage(): AppLanguage {
   return DEFAULT_LANGUAGE;
 }
 
+const persistedLang = getPersistedLanguage();
+
 i18n.use(initReactI18next).init({
   resources: {
     'zh-CN': { translation: zhCN },
-    'en-US': { translation: enUS },
   },
-  lng: getPersistedLanguage(),
+  // Start with zh-CN if the persisted language (en-US) hasn't been loaded yet.
+  // en-US is fetched asynchronously below and the language is switched once ready.
+  lng: persistedLang === 'en-US' ? 'zh-CN' : persistedLang,
   fallbackLng: 'zh-CN',
   interpolation: {
     escapeValue: false, // React already escapes
@@ -42,6 +62,15 @@ i18n.use(initReactI18next).init({
   },
 });
 
+// If the user previously selected en-US, load it asynchronously then switch.
+// There is a brief flash of Chinese before English loads — acceptable trade-off
+// for a ~290 kB reduction in the main entry chunk.
+if (persistedLang === 'en-US') {
+  void ensureLocaleLoaded('en-US').then(() => {
+    i18n.changeLanguage('en-US');
+  });
+}
+
 // Sync <html lang="..."> with current language
 i18n.on('languageChanged', (lng) => {
   if (typeof document !== 'undefined') {

frontend/src/store/index.ts

@@ -29,9 +29,10 @@ export const useLanguageStore = create<LanguageState>()(
     (set) => ({
       language: "zh-CN" as AppLanguage,
       setLanguage: (lang) => {
-        // 同步切换 i18next 实例
+        // Ensure the target locale bundle is loaded (en-US is lazy) before switching.
         import("@/i18n")
-          .then(({ default: i18n }) => {
+          .then(async ({ default: i18n, ensureLocaleLoaded }) => {
+            await ensureLocaleLoaded(lang);
             i18n.changeLanguage(lang);
           })
           .catch(() => {