Tint chat composer background (#1921)

## Summary
- give the chat composer a subtle custom background and apply it across
the full area drawn

<img width="1008" height="718" alt="composer-bg"
src="https://github.com/user-attachments/assets/4b0f7f69-722a-438a-b4e9-0165ae8865a6"
/>

- update turn interrupted to be more human readable
<img width="648" height="170" alt="CleanShot 2025-08-06 at 22 44 47@2x"
src="https://github.com/user-attachments/assets/8d35e53a-bbfa-48e7-8612-c280a54e01dd"
/>

## Testing
- `cargo test --all-features` *(fails: `let` expressions in
`core/src/client.rs` require newer rustc)*
- `just fix` *(fails: `let` expressions in `core/src/client.rs` require
newer rustc)*

------
https://chatgpt.com/codex/tasks/task_i_68941f32c1008322bbcc39ee1d29a526

codex-rs/core/src/codex.rs

@@ -688,7 +688,7 @@ impl AgentTask {
             let event = Event {
                 id: self.sub_id,
                 msg: EventMsg::Error(ErrorEvent {
-                    message: "Turn interrupted".to_string(),
+                    message: " Turn interrupted".to_string(),
                 }),
             };
             let tx_event = self.sess.tx_event.clone();