fix(web): adjust tablet font scaling

This makes two corrections:

1. Removed an obsolete test for screen size based on dpi, which is
   unreliable, meant that we had been treating many Android tablets as
   phones (domManager.ts).
2. Font scale for Android tablets had some incorrect assumptions around
   keyboard height and devicePixelRatio, which I was able to simplify
   into a fixed ratio, and was much better across all devices I
   tested (however, all emulated in Chrome).