on-auth-change.ts

useEffect(() => {
  const { data: authListener } = supabase.auth.onAuthStateChange(async (event, session) => {
    await fetch('/api/set-auth-cookie', {
      method: 'POST',
      headers: new Headers({ 'content-Type': 'application/json' }),
      body: JSON.stringify({ session, event }),
    });
  });

  return () => {
    authListener?.unsubscribe();
  };
}, []);