private void btnDangNhap_Click(object sender, EventArgs e)
        {
            frmDangNhap frm = new frmDangNhap();

            frm.ShowDialog();
        }
        private void pn1_Paint(object sender, PaintEventArgs e)
        {
            frmDangNhap frm = new frmDangNhap();

            frm.ShowDialog();
        }