Lier un JPanel à un JFrame (setContentPan)

public class Fenetre extends JFrame {

    private EcranDepart dep = new EcranDepart();

    public static void main(String[] args) {
        Fenetre f = new Fenetre();
    }

    public Fenetre() {
        this.setTitle("Fenetre");
        this.setSize(800, 600);
        this.setLocationRelativeTo(null);
        this.setResizable(false);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setContentPane(dep);
        this.setVisible(true);
    }
}

class EcranDepart extends JPanel {
    
    public JPanel pan;
    public JPanel pantop; 
    public JPanel panmid; 
    public JPanel panbot; 
    
    public JButton connect;
    public JButton sub;
    public JButton info;
    
    public EcranDepart() {
        pan = new JPanel();
        pan.setLayout(new BorderLayout());
        
            pantop = new JPanel();
            pantop.setPreferredSize(new Dimension(800, 0));
            pan.add(pantop, BorderLayout.NORTH);
            
            panmid = new JPanel();
            panmid.setBackground(Color.red);
            pan.add(panmid, BorderLayout.CENTER);
                
                connect = new JButton("Se connecter");
                connect.setPreferredSize(new Dimension(400, 100));
                sub = new JButton("S'incrire");
                sub.setPreferredSize(new Dimension(400, 100));
                panmid.add(connect);
                panmid.add(sub);
            
            panbot = new JPanel();
            panbot.setBackground(Color.white);         
            panbot.setPreferredSize(new Dimension(800, 50));
            panbot.setLayout(new BorderLayout());
            pan.add(panbot, BorderLayout.SOUTH);
            
                info = new JButton("i");
                info.setPreferredSize(new Dimension(50,50));
                panbot.add(info, BorderLayout.EAST);
    }
}