keep a 1 cell margin at the right edge of the screen in the composer
(and in the user message in history).
this lets us print clear-to-EOL 1 char before the end of the line in
history, so that resizing the terminal will keep the background color
(at least in iterm/terminal.app). it also stops the cursor in the
textarea from floating off the right edge.
---------
Co-authored-by: joshka-oai <joshka@openai.com>
9be704a934