update composer + user message styling (#4240)

Changes:

- the composer and user messages now have a colored background that
stretches the entire width of the terminal.
- the prompt character was changed from a cyan `▌` to a bold `›`.
- the "working" shimmer now follows the "dark gray" color of the
terminal, better matching the terminal's color scheme

| Terminal + Background        | Screenshot |
|------------------------------|------------|
| iTerm with dark bg | <img width="810" height="641" alt="Screenshot
2025-09-25 at 11 44 52 AM"
src="https://github.com/user-attachments/assets/1317e579-64a9-4785-93e6-98b0258f5d92"
/> |
| iTerm with light bg | <img width="845" height="540" alt="Screenshot
2025-09-25 at 11 46 29 AM"
src="https://github.com/user-attachments/assets/e671d490-c747-4460-af0b-3f8d7f7a6b8e"
/> |
| iTerm with color bg | <img width="825" height="564" alt="Screenshot
2025-09-25 at 11 47 12 AM"
src="https://github.com/user-attachments/assets/141cda1b-1164-41d5-87da-3be11e6a3063"
/> |
| Terminal.app with dark bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 22 AM"
src="https://github.com/user-attachments/assets/93fc4781-99f7-4ee7-9c8e-3db3cd854fe5"
/> |
| Terminal.app with light bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 46 04 AM"
src="https://github.com/user-attachments/assets/19bf6a3c-91e0-447b-9667-b8033f512219"
/> |
| Terminal.app with color bg | <img width="577" height="367"
alt="Screenshot 2025-09-25 at 11 45 50 AM"
src="https://github.com/user-attachments/assets/dd7c4b5b-342e-4028-8140-f4e65752bd0b"
/> |

codex-rs/tui/src/bottom_pane/mod.rs

@@ -150,7 +150,9 @@ impl BottomPane {
                 let status_height = self
                     .status
                     .as_ref()
-                    .map_or(0, |status| status.desired_height(area.width));
+                    .map_or(0, |status| status.desired_height(area.width))
+                    .min(area.height.saturating_sub(1));
+
                 Layout::vertical([Constraint::Max(status_height), Constraint::Min(1)]).areas(area)
             }
         }
@@ -616,7 +618,7 @@ mod tests {
 
         // Composer placeholder should be visible somewhere below.
         let mut found_composer = false;
-        for y in 1..area.height.saturating_sub(2) {
+        for y in 1..area.height {
             let mut row = String::new();
             for x in 0..area.width {
                 row.push(buf[(x, y)].symbol().chars().next().unwrap_or(' '));