[changes]

-made the game run in the background because there were some undetectable input-related malfunctions.

-changed some texts in the difficulty selection

[bug fixes]

-fixed some malfunctioning UI

-fixed a bug where a player can be invincible from hazardous spikes after hitting a projectile. 

-fixed a bug where you can spend other skill slot's mana while quick-swapping the skill set.

-fixed a bug where you can shoot a skill different from the chosen one.