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/page.tsx

@@ -0,0 +1,18 @@
+import { cookies } from 'next/headers'
+import DashboardPage from './DashboardPage'
+
+export default async function Home() {
+  const cookieStore = await cookies()
+  const userCookie = cookieStore.get('user')?.value
+
+  let user = null
+  if (userCookie) {
+    try {
+      user = JSON.parse(userCookie)
+    } catch {
+      // ignore
+    }
+  }
+
+  return <DashboardPage user={user} />
+}