Don't remove event listeners from nav buttons

They should only be added once anyway.
Fix box/flex rules in CSS for Firefox
Turn on select for input elements

Turning off select for all elements made it impossible to enter
high scores.
Use correct selector to make inputs selectable
Align puzzle without hbox

Firefox doesn't seem to render display:box correctly if there
is only one child inside the box. Use relative sizing and
margins to do centering instead.