Header text gets cropped off vertically on mobile

[JoshuaKGoldberg]

Overview

I noticed in #24's video report that the header looks like this:

The page should use a CSS media query to make the header's font size smaller on mobile devices. That way the full Choose Your Framework text will always be visible.

As with #26, src/index.css is the place to write styles. They'll need to be copied to lib/index.css to be seen when running the game locally from lib/index.html.