Final boss quality of life improvements update

It's now possible to bring up the pause menu and rebind keys during the core phase of the final boss. The game won't actually pause since it makes no sense storywise.

An icon will appear at the edges of the screen whenever the core leaves the screen. This will help players determine its location even when they can't see it

Made the core telegraph one of its attacks so it's easier to predict and dodge

Fixed a bug causing a save file softlock when players quit the game after finishing the idle game.