Really interesting idea for a puzzle game and I especially like the extra touches you added like the snap sound when the pieces move.
The game also looks quite aesthetically pleasing between all of the menus and the board itself.
Tho only minor change i would have recommend would be to allow the player to access a menu allowing restarting the game and returning to the main menu when pressing a key such as escape. this is very minor and me being picky
If the player closes the "invalid square" warning using the cross instead of the "ok" button the game softlocks with the player unable to make any move.