Fix: prevent resize listener accumulation in makeBackground

[severe77]

Summary

This PR fixes a resize listener accumulation issue in js/turtles.js
inside the Turtles.TurtlesView class.

Problem

In the makeBackground() method, a new window.resize event listener
was being registered each time the method was called.

Since the resize handler reference was not preserved across calls,
previously registered listeners were not removed before adding new ones,
leading to multiple active resize listeners.

Because makeBackground() may run multiple times during normal
canvas updates, this could result in multiple resize listeners
being attached.

Solution

• Defined a named resizeHandler function inside makeBackground().

• Stored the handler reference on the instance (this._resizeHandler).

• Before registering a new listener, the existing one (if present)
  is removed using:

  window.removeEventListener("resize", this._resizeHandler);

[github-actions]

✅ All Jest tests passed! This PR is ready to merge.

[kartikktripathi]

This looks good to me. The resize listener accumulation issue is addressed properly by storing the handler reference and ensuring any existing listener is removed before registering a new one.
The fix is clean, scoped, and improves lifecycle stability without altering existing behaviour.

[severe77]

@walterbender could you please take a look at this pr when you have some time?