fix: 实现中英文语言切换功能

- 新建 LocaleSwitcher 组件支持语言切换
- 使用 usePathname 保持当前页面路径
- 点击切换时自动更新 URL 语言前缀

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/app/[locale]/layout.tsx

@@ -1,6 +1,7 @@
 import { notFound } from "next/navigation"
 import { setRequestLocale } from 'next-intl/server'
 import Link from "next/link"
+import { LocaleSwitcher } from "@/components/locale/LocaleSwitcher"
 
 const locales = ['zh', 'en']
 
@@ -72,9 +73,7 @@ export default async function LocaleLayout({
 
             {/* Right side */}
             <div className="hidden md:flex items-center space-x-4">
-              <button className="font-display text-sm font-bold hover:opacity-70">
-                中文 / EN
-              </button>
+              <LocaleSwitcher currentLocale={locale} />
               <Link
                 className="bg-white dark:bg-surface-dark border-2 border-black dark:border-white px-4 py-2 font-display text-sm font-bold shadow-neo-sm hover:shadow-none hover:translate-x-[2px] hover:translate-y-[2px] transition-all"
                 href="#"