Cancel button for tile upgrade in rest area and buffed final boss HP — Tiles & Vials Steam update #7187909

I found that it was possible to soft-lock yourself if you had a fully maxed out inventory and then went to the tile upgrade section in the rest area. So I added a cancel button to that, so you can easily back out of it. Plus I figured it's nice to be able to choose something else if you made a mistake. I've also seen pretty much everyone just completely destroy the final boss because by the time they get there, they're very powerful. So I increased his HP from 300 to 999. Let me know if you can still manage to kill him in a single turn now!!