i18n: replace 'CMS' with '后台管理' in Chinese interface

- Update admin layout title from 'CMS' to '后台管理'
- Update terms page service description from 'CMS' to '后台管理系统'
- Improve Chinese localization consistency

src/app/admin/layout.tsx

@@ -66,7 +66,7 @@ export default function AdminLayout({
     <div className="min-h-screen bg-gray-50">
       <div className="lg:hidden fixed top-0 left-0 right-0 z-40 bg-white border-b border-gray-200 px-4 py-3 flex items-center justify-between">
         <Link href="/admin" className="text-xl font-bold text-[#C41E3A]">
-          睿新致遠 CMS
+          睿新致遠 后台管理
         </Link>
         <button
           onClick={() => setSidebarOpen(!sidebarOpen)}
@@ -91,7 +91,7 @@ export default function AdminLayout({
         <div className="h-full flex flex-col">
           <div className="h-16 flex items-center px-6 border-b border-gray-200">
             <Link href="/admin" className="text-xl font-bold text-[#C41E3A]">
-              睿新致遠 CMS
+              睿新致遠 后台管理
             </Link>
           </div>