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.

2 files changed, 7 insertions, 3 deletions

diff --git a/web/demo/overlay/root/.config/slosh/config.kdl b/web/demo/overlay/root/.config/slosh/config.kdl
index 513f9c6..dd5af63 100644
--- a/web/demo/overlay/root/.config/slosh/config.kdl
+++ b/web/demo/overlay/root/.config/slosh/config.kdl
@@ -6,7 +6,10 @@
 // thing to ship to a browser.
 
 // The palette. Every theme the repo ships is in themes/ right here, so
-// swapping the whole look is editing one word and saving:
+// swapping the whole look is editing one word and saving -- or pressing
+// `C-a t`, which lists them and puts each one on as you arrow through it
+// (`C-s` in that list writes the name back into this file, `esc` puts back
+// what you were wearing):
 //
 //   sl0p        hot pink on near-black (this one)
 //   phosphor    a green terminal remembering being a VT220
diff --git a/web/demo/overlay/root/demo.layout b/web/demo/overlay/root/demo.layout
index 0d29689..7515124 100644
--- a/web/demo/overlay/root/demo.layout
+++ b/web/demo/overlay/root/demo.layout
@@ -37,10 +37,11 @@ layout {
         // command that exited would leave a corpse; this one only ends when
         // the pane closes), and the float's shadow plus everyone else's
         // dim_unfocused make it read as the dialog it is.
-        // Size is the text's (56 columns of it, plus the frame); position is
+        // Size is the text's (56 columns and 21 rows of it, plus the frame);
+        // position is
         // deliberately absent, so the float takes the centred default at
         // whatever size the browser gave the terminal. A smaller screen
         // clamps the rect and nothing is owed.
-        pane floating=true focus=true w=62 h=22 purpose="demo:hello" command="cat /etc/welcome; exec cat"
+        pane floating=true focus=true w=62 h=23 purpose="demo:hello" command="cat /etc/welcome; exec cat"
     }
 }