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/callback/page.tsx

@@ -0,0 +1,74 @@
+'use client'
+
+import { Suspense } from 'react'
+import { useEffect, useState } from 'react'
+import { useSearchParams } from 'next/navigation'
+import { Stack, Text, Loader } from '@mantine/core'
+
+function CallbackContent() {
+  const searchParams = useSearchParams()
+  const [error, setError] = useState<string | null>(null)
+
+  useEffect(() => {
+    const code = searchParams.get('sso_code')
+    if (!code) {
+      setError('缺少 sso_code 参数')
+      return
+    }
+
+    // 用 code 调用本应用的 /api/callback 换取用户信息
+    fetch('/api/callback', {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ code }),
+    })
+      .then(async (res) => {
+        if (!res.ok) {
+          const data = await res.json()
+          throw new Error(data.error || '认证失败')
+        }
+        // 成功 -> 跳转到首页
+        window.location.href = '/'
+      })
+      .catch((err) => {
+        setError(err instanceof Error ? err.message : '认证失败')
+      })
+  }, [searchParams])
+
+  if (error) {
+    return (
+      <Stack align="center" justify="center" h="100vh" gap="md">
+        <Text size="lg" c="red">认证失败</Text>
+        <Text size="sm" c="dimmed">{error}</Text>
+        <Text
+          size="sm"
+          c="brand"
+          style={{ cursor: 'pointer' }}
+          onClick={() => (window.location.href = '/')}>
+          返回首页重试
+        </Text>
+      </Stack>
+    )
+  }
+
+  return (
+    <Stack align="center" justify="center" h="100vh" gap="md">
+      <Loader size="lg" />
+      <Text size="sm" c="dimmed">正在验证 SSO 登录...</Text>
+    </Stack>
+  )
+}
+
+export default function CallbackPage() {
+  return (
+    <Suspense
+      fallback={
+        <Stack align="center" justify="center" h="100vh" gap="md">
+          <Loader size="lg" />
+          <Text size="sm" c="dimmed">正在验证 SSO 登录...</Text>
+        </Stack>
+      }>
+      <CallbackContent />
+    </Suspense>
+  )
+}