display check *after* piece animation finishes

[sohamkorade]

Currently, the animation behavior follows the pattern shown on the left. However, I suggest that the behavior depicted on the right would be more natural, particularly when navigating backward in a PGN game.

Left (current): Displays check as soon as the piece leaves its original square.
Right (proposed): Displays check after the piece reaches its destination square.

Code changes required:
Exclude the appending of a new square with the check class until the piece animation has finished.

Your feedback is greatly appreciated.