Handle Ctrl+C quit when idle (#1402)

## Summary
- show `Ctrl+C to quit` hint when pressing Ctrl+C with no active task
- exiting with Ctrl+C if the hint is already visible
- clear the hint when tasks begin or other keys are pressed


https://github.com/user-attachments/assets/931e2d7c-1c80-4b45-9908-d119f74df23c



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https://chatgpt.com/s/cd_685ec8875a308191beaa95886dc1379e

Fixes #1245

codex-rs/tui/src/app.rs

@@ -11,7 +11,6 @@ use crate::slash_command::SlashCommand;
 use crate::tui;
 use codex_core::config::Config;
 use codex_core::protocol::Event;
-use codex_core::protocol::Op;
 use color_eyre::eyre::Result;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
@@ -193,10 +192,11 @@ impl<'a> App<'a> {
                             modifiers: crossterm::event::KeyModifiers::CONTROL,
                             ..
                         } => {
-                            // Forward interrupt to ChatWidget when active.
                             match &mut self.app_state {
                                 AppState::Chat { widget } => {
-                                    widget.submit_op(Op::Interrupt);
+                                    if widget.on_ctrl_c() {
+                                        self.app_event_tx.send(AppEvent::ExitRequest);
+                                    }
                                 }
                                 AppState::Login { .. } | AppState::GitWarning { .. } => {
                                     // No-op.