feat: P0a i18n 基础设施 + UI 语言切换

- 创建 i18n 模块 (provider + useT hook + useLang hook)
- 中/英翻译文件 (zh.ts / en.ts)，覆盖沙盒/学习/会员/对比等核心页面
- LanguageProvider 包装 layout，支持 localStorage 持久化
- Header 添加语言切换按钮 (中<>EN)
- AGENTS.md 新增国际化约定
- opencode 源码克隆到 /tmp/opencode-source/ 供参考

frontend/src/components/layout/header.tsx

@@ -6,9 +6,10 @@ import { usePathname } from 'next/navigation';
 import { Button } from '@/components/ui/button';
 import { Input } from '@/components/ui/input';
 import { ThemeToggle } from '@/components/ui/theme-toggle';
-import { Search, Menu, X, Bell } from 'lucide-react';
+import { Search, Menu, X, Bell, Languages } from 'lucide-react';
 import { apiFetch } from '@/lib/auth';
 import { useAuth } from '@/lib/auth-context';
+import { useLang } from '@/i18n';
 
 const navItems = [
   { href: '/', label: '首页' },
@@ -60,6 +61,7 @@ export function Header() {
   const [mobileOpen, setMobileOpen] = useState(false);
   const [scrolled, setScrolled] = useState(false);
   const { isLoggedIn, logout } = useAuth();
+  const { lang, setLang } = useLang();
 
   function isActive(href: string) {
     if (href === '/') return pathname === '/';
@@ -114,6 +116,10 @@ export function Header() {
               />
             </form>
             <ThemeToggle />
+            <button onClick={() => setLang(lang === 'zh' ? 'en' : 'zh')}
+              className="p-2 rounded-lg text-muted-foreground hover:text-foreground hover:bg-accent transition-colors text-xs font-medium">
+              {lang === 'zh' ? 'EN' : '中'}
+            </button>
           </div>
 
           <div className="hidden md:flex items-center gap-2">
@@ -145,6 +151,10 @@ export function Header() {
 
           <div className="flex md:hidden items-center gap-2">
             <ThemeToggle />
+            <button onClick={() => setLang(lang === 'zh' ? 'en' : 'zh')}
+              className="p-2 rounded-lg text-muted-foreground hover:text-foreground hover:bg-accent transition-colors text-xs font-medium">
+              {lang === 'zh' ? 'EN' : '中'}
+            </button>
             <Button
               variant="ghost"
               size="icon"