private void ButtonLogin_Click(object sender, RoutedEventArgs e)
        {
            string account = textboxAccountNumber.Text;
            string pin     = textboxPin.Password;

            token = bank.Authorize(account, pin);
            if (token.Length == 0)
            {
                labelLoginError.Content = "Podane dane logowania są niepoprawne";
            }
            else
            {
                ClearAll();
                gridLogin.Visibility       = Visibility.Hidden;
                gridClientPanel.Visibility = Visibility.Visible;
                RefreshBalance();
                labelCurrency.Content = bank.GetCurrency(token);
            }
        }