[frogend] Replace ch units to prevent layout shift on page load

[f0x52]

We use the ch CSS unit in a few places, most notably the main page column width. It's relative to the current font's width of the 0 character, which has the fun side-effect that it changes value during page load, when the font gets swapped from the fallback sans-serif to the asset loaded Noto Sans.
I picked rem values instead that are roughly equivalent in apparent size, so there's no more page-load shift

Checklist

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