private void LoginButton_Click(object sender, RoutedEventArgs e)
        {
            ManagerClient client = new ManagerClient();

            if (client.Login(loginBox.Text, passwordBox.Password) == true)
            {
                MainWindowViewModel.db = client;
                this.Hide();
                new MainWindow(loginBox.Text).ShowDialog();
                this.Close();
            }
            else
            {
                MessageBox.Show("Nieprawidłowy login/hasło");
            }
        }