chore(apps/account): update brand headline to AI-native positioning

Replace the legacy 'post-SaaS operating system' tagline on the auth
shell with the AI-native backend message, matching the documented
positioning in content/docs/index.mdx.

- auth-shell.tsx default copy
- en.json brandHeadline
- zh-CN.json brandHeadline

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: hotlong

apps/account/src/components/auth/auth-shell.tsx

@@ -86,7 +86,7 @@ export function AuthShell({
           <h2 className="text-balance text-3xl font-semibold leading-tight tracking-tight md:text-4xl">
             {headline ??
               t('auth.login.brandHeadline', {
-                defaultValue: 'The post-SaaS operating system for your team.',
+                defaultValue: 'The AI-native backend for business software.',
               })}
           </h2>
           <p className="text-balance text-base/relaxed text-white/80">

apps/account/src/i18n/locales/en.json

@@ -248,7 +248,7 @@
       "signUp": "Sign up",
       "welcomeToast": "Welcome back",
       "failed": "Sign in failed",
-      "brandHeadline": "The post-SaaS operating system for your team.",
+      "brandHeadline": "The AI-native backend for business software.",
       "brandSubline": "One identity, every workspace. Sign in to manage your account, organizations and connected apps."
     },
     "register": {

apps/account/src/i18n/locales/zh-CN.json

@@ -248,7 +248,7 @@
       "signUp": "注册",
       "welcomeToast": "欢迎回来",
       "failed": "登录失败",
-      "brandHeadline": "面向团队的后 SaaS 操作系统。",
+      "brandHeadline": "面向 AI 原生业务软件的元数据后端。",
       "brandSubline": "一个身份，连接所有工作区。登录即可管理账户、组织与已授权应用。"
     },
     "register": {