feat(usercenter/i18n): 6 语(加 日/韩/俄/西)+ 默认英文

- lib/i18n.ts: Lang/Entry 扩 6 语;STRINGS 130 条补 ja/ko/ru/es;makeT 回退英文
- lib/api/errors.ts: ERROR_TEXT 14 条补 4 语;bilingual 改为 code 本地字典优先
  → 英文字典 → 后端 message → 通用兜底
- lib/theme.tsx: 默认语种 zh→en;localStorage 白名单扩 6 语;setAttribute('lang') 直用 lang
- components/shared.tsx: LangSeg 段控 → 原生 select 6 语下拉
- app/layout.tsx: 默认壳 html lang zh→en;title/desc 改英文
- next build 通过(5 页静态);tsc 干净;无红线词

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

web/usercenter/lib/theme.tsx

@@ -18,7 +18,7 @@ const LANG_KEY = 'pg_uc_lang';
 const THEME_KEY = 'pg_uc_theme';
 
 export function UIProvider({ children }: { children: React.ReactNode }) {
-  const [lang, setLangState] = useState<Lang>('zh');
+  const [lang, setLangState] = useState<Lang>('en'); // 默认英文（国际化默认语种）
   const [theme, setThemeState] = useState<Theme>('light');
 
   // 挂载后读取持久化(避免 hydration 不一致)
@@ -26,7 +26,7 @@ export function UIProvider({ children }: { children: React.ReactNode }) {
     try {
       const l = window.localStorage.getItem(LANG_KEY) as Lang | null;
       const t = window.localStorage.getItem(THEME_KEY) as Theme | null;
-      if (l === 'zh' || l === 'en') setLangState(l);
+      if (l && (['zh', 'en', 'ja', 'ko', 'ru', 'es'] as Lang[]).includes(l)) setLangState(l);
       const initial: Theme = t === 'dark' || t === 'light' ? t : window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light';
       setThemeState(initial);
     } catch {
@@ -36,7 +36,7 @@ export function UIProvider({ children }: { children: React.ReactNode }) {
 
   useEffect(() => {
     document.documentElement.setAttribute('data-theme', theme);
-    document.documentElement.setAttribute('lang', lang === 'zh' ? 'zh' : 'en');
+    document.documentElement.setAttribute('lang', lang);
   }, [theme, lang]);
 
   const setLang = useCallback((l: Lang) => {