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/globals.css

@@ -0,0 +1,8 @@
+html, body {
+  overflow-x: hidden;
+}
+* {
+  box-sizing: border-box;
+  padding: 0;
+  margin: 0;
+}