Prevent the event!

- Switched from keypress to keydown for detecting key presses and use
event.preventDefault() to hinder the browser from scrolling the window
when playing.
- Changed the controls to use arrow keys instead of asdw, and
focus/unfocus from § to tab.
- Battle meter is now hidden by default and becomes visible (display:
block/none) when Battle Mode is triggered.
- Minor cosmetic fixes.