chore: initial commit of external-app SSO consumer

Next.js sample application that integrates with the mock SSO service:
redirects to sso-mock for login, receives the one-time sso_code on its
/callback route, and exchanges it for user info to establish a session.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/api/logout/route.ts

@@ -0,0 +1,15 @@
+import { NextResponse } from 'next/server'
+
+/**
+ * 退出登录
+ * 清除所有认证相关 Cookie。
+ */
+export async function POST() {
+  const response = NextResponse.json({ success: true })
+
+  response.cookies.set('user', '', { path: '/', maxAge: 0 })
+  response.cookies.set('token', '', { path: '/', maxAge: 0 })
+  response.cookies.set('refresh_token', '', { path: '/', maxAge: 0 })
+
+  return response
+}