gs-details-page: use tabular figures for install progress

makes text less wobbly as percentage changes

I couldn't get latest main to build on my system, so I tested this on 42.2 and cherry-picked it onto main.

See https://blog.horizon-eda.org/misc/2020/02/19/ui.html for more detail on why this matters.

We're still missing the padding for <10%, but this change gets us mostly there.