HIGHLY UNFINISHED CODE

:|

This is kind of a break of purity, because it reads
`window.offsetWidth`, but this is the lowest-overhead way to
automatically recalculate the leftRoom width whenever PS updates
(i.e. whenever leftRoom or rightRoom change).

Considering all the individual panels start with `panel-`, this should
make it easier to find and in general be a more intuitive name.

All other tsx files will start with `panel-`, putting them together in
one place when sorting alphabetically.

Room layout code is now better commented and more readable.

There's still a lot more to do, but this is a start

Unlike the old router, this one has no dependencies, and also tracks
left/right panel state, for more accurate back/forward button support.

It also sets the URL to the last selected panel, instead of always
using the left panel.

The actual connect function is commented out, because there's no reason
for it to do anything. But it can actually connect to a server now.

This at least lets me test some router features without pushing it to
the live server.