Store the board size as static variables

src/js/Game.js

@@ -7,6 +7,16 @@ import { State } from "./State.js";
  */
 export class Game
 {
+    /**
+     * The width of the board.
+     */
+    static #width = 7;
+
+    /**
+     * The height of the board.
+     */
+    static #height = 6;
+
     /**
      * The state of the game.
      *
@@ -74,9 +84,9 @@ export class Game
         let board = document.getElementById(this.id);
         board.innerHTML = "";
 
-        for (let i = 0; i < 6; i++)
+        for (let i = 0; i < Game.#height; i++)
         {
-            for (let j = 0; j < 7; j++)
+            for (let j = 0; j < Game.#width; j++)
             {
                 /** @type {Node[]} */
                 let children = [];