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/app/layout-client.tsx

@@ -0,0 +1,24 @@
+'use client'
+
+import { I18nProvider } from '@/i18n'
+import { ThemeProvider } from '@/components/providers/theme-provider'
+import { AuthProvider } from '@/lib/auth-context'
+import { Toaster } from '@/components/ui/sonner'
+import { Header } from '@/components/layout/header'
+import { Footer } from '@/components/layout/footer'
+import type { ReactNode } from 'react'
+
+export function RootLayoutClient({ children }: { children: ReactNode }) {
+  return (
+    <ThemeProvider attribute="class" defaultTheme="system" enableSystem disableTransitionOnChange>
+      <AuthProvider>
+        <I18nProvider>
+          <Header />
+          <main className="flex-1">{children}</main>
+          <Footer />
+          <Toaster richColors closeButton />
+        </I18nProvider>
+      </AuthProvider>
+    </ThemeProvider>
+  )
+}