Hotfixes and Undo Button

Special thanks to Nievano on Discord for the bug reports and suggestions!

• Fixed summoning pieces in single player.
• Fixed health bars in multiplayer combat.
• Fixed Scorpion gaining a charge from being hit by its own reflected projectile.
• Added button for undoing item purchase.

As of now, the undo button only resets your item purchase. If you move a piece, your turn is immediately sent. Undoing movement will require clicking a button each turn to confirm. Let us know what you think.