webdemo: the picker in the machine, and a guest that proves its themes

The demo image was cut before any of this existed: an old guest binary with
no `C-a t` in it, and a config whose theme was a pasted palette. Rebuilt --
guest binary, root filesystem, site -- so the thing a visitor lands in is the
thing the front page is now claiming.

The overlay already kept every shipped theme in ~/.config/slosh/themes (they
are symlinks into contrib/, copied by content because a symlink in an ext2
image would dangle), which turns out to be exactly the directory the picker
looks in first -- and in the guest it is the *only* one, since /usr/bin/slosh
has no share/slosh beside it. So `boot-check` now asks the guest itself:
`{"cmd":"theme"}` over --script, expecting the worn theme and the shipped list
back. It failed honestly the first time, for the right reason (no HOME in a
phase that boots straight to /bin/sh, so no config, so no themes/ beside it),
which is the kind of failure a check is for.

The welcome float grew a fourth line -- `C-a t`, every theme, worn as you
browse -- and a row to hold it; the motd and the config's own comments say the
same. A visitor who reads three lines and presses one key should land on the
feature the page just sold them.

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