[TUI] improve bash mode UI

What variant of Codex are you using?

CLI

What feature would you like to see?

I don't use bash mode often, but when I do, I love to see a visual feedback to know that I am in bash mode.

Warp used to do it well before switching to the /agent command, and Claude Code does this really well:

| Claude Code | Codex |
|--------|--------|
| <img width="165" height="77" alt="Image" src="https://github.com/user-attachments/assets/6e8e87d2-708c-4e36-a761-e172ee1f005b" /> | <img width="150" height="78" alt="Image" src="https://github.com/user-attachments/assets/dffa7460-b8e2-4d7f-a618-d294f5b48239" /> |

I think that changing > to ! and updating the input background color slightly could go a long way.

I'd love to send a PR. Let me know!