Show the list and the conversation at once on a wide window

The app is laid out for a phone: the list and the chat take turns, and a
desktop window gets the same single column with margins either side. Past
900pt there is room for both, so the chats screen becomes two panes — the
list at 320, the conversation in what is left — and opening a channel
there is a change of `current` rather than a change of screen. Discover
and settings stay whole screens: they are somewhere else, not the other
half of here.

Nothing reported a window's size, so the backend writes it onto the
window node the way an entry writes back its text, and a timer polls it
into a cell. A poll rather than an event: only what a component derefs
re-renders, so the layout follows a drag of the window's edge without
every frame touching the tree.

Two things the panes needed from vidya. `:fill-height`, because a column
in a row is otherwise as tall as the row — one button, at the moment it
is placed — and both the list and the backlog size themselves against the
height they are handed. And `:wrap false`, because an `:hbox` wraps: the
conversation asking for a few points more than were left was painted on a
row below the list, off the bottom of the window, which reads exactly
like a conversation that has gone missing. So only the list is given a
width now, and the conversation takes the remainder rather than the
window's width minus a guess at the list's.

The glimmer pin moves for the other half of that bug: replacing a native
node with one of another tag left every watcher under it subscribed, so
swapping the layouts shredded the chat — one component's props painted
onto another's widget.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>