Pin the backend at the scroll and column fixes the chat screen needed

The backlog opened on the oldest line and stayed there, because a viewport's
offset lived in a prop and a re-render clears props — so the position was lost
every time a message arrived. It is remembered under :scroll-key now, sticky
ones follow their own bottom, and a reader who scrolls up keeps their place.

With that visible, two more: a column could not shrink at all, so a backlog
taller than the screen painted the compose bar past the bottom edge; and a
width-request was a floor rather than a width, so the chats list was as wide as
the longest thing anyone had said in it and the conversation beside it got ten
cells. Both are what the split view looks like, rather than notes about it.

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