tui: reserve 1 cell right margin for composer and user history (#5026)

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>

codex-rs/tui/src/snapshots/codex_tui__history_cell__tests__user_history_cell_wraps_and_prefixes_each_line_snapshot.snap

@@ -3,6 +3,6 @@ source: tui/src/history_cell.rs
 expression: rendered
 ---
 › one two
-  three four
-  five six
-  seven
+  three
+  four five
+  six seven