Fix main menu alignment.

It wasn't centered.

I also tweaked some other aspects of placement and padding... it should make a little more sense now.

That hardcoded 2 character offset wasn't actually being applied, altho it was being used for the spacing calculation. I set it to zero, but made the interior spacing also apply to the ends. IMO it looks better at large terminal sizes now. Before the menu was weirdly spaced out, but now it more evenly spreads across the line underneath it.