update GamePanel

src/main/java/GamePanel.java

@@ -1,3 +1,11 @@
+import javax.swing.*;
 import java.awt.*;
 
-public class GamePanel {
+public class GamePanel extends JPanel implements Runnable {
+    static final int FRAME_WIDTH = 1000;
+    static final int FRAME_HEIGHT = (int) (FRAME_WIDTH * 0.5);
+    static final int BALL_DIAMETER = 26;
+    static final int PADDLE_WIDTH = 25;
+    static final int PADDLE_HEIGHT = 100;
+    static final Dimension SCREEN = new Dimension(FRAME_WIDTH, FRAME_HEIGHT);
+}