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>

next.config.ts

@@ -0,0 +1,7 @@
+import type { NextConfig } from 'next'
+
+const nextConfig: NextConfig = {
+  output: 'standalone',
+}
+
+export default nextConfig