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/history_cell.rs

@@ -114,7 +114,11 @@ impl HistoryCell for UserHistoryCell {
     fn display_lines(&self, width: u16) -> Vec<Line<'static>> {
         let mut lines: Vec<Line<'static>> = Vec::new();
 
-        let wrap_width = width.saturating_sub(LIVE_PREFIX_COLS);
+        let wrap_width = width
+            .saturating_sub(
+                LIVE_PREFIX_COLS + 1, /* keep a one-column right margin for wrapping */
+            )
+            .max(1);
 
         let style = user_message_style();
 
@@ -125,7 +129,8 @@ impl HistoryCell for UserHistoryCell {
                 .map(|l| Line::from(l).style(style))
                 .collect::<Vec<_>>(),
             // Wrap algorithm matches textarea.rs.
-            RtOptions::new(wrap_width as usize).wrap_algorithm(textwrap::WrapAlgorithm::FirstFit),
+            RtOptions::new(usize::from(wrap_width))
+                .wrap_algorithm(textwrap::WrapAlgorithm::FirstFit),
         );
 
         lines.push(Line::from("").style(style));