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



------
https://chatgpt.com/s/cd_685ec8875a308191beaa95886dc1379e

Fixes #1245

codex-rs/tui/src/chatwidget.rs

@@ -138,6 +138,7 @@ impl ChatWidget<'_> {
     }
 
     pub(crate) fn handle_key_event(&mut self, key_event: KeyEvent) {
+        self.bottom_pane.clear_ctrl_c_quit_hint();
         // Special-case <Tab>: normally toggles focus between history and bottom panes.
         // However, when the slash-command popup is visible we forward the key
         // to the bottom pane so it can handle auto-completion.
@@ -244,6 +245,7 @@ impl ChatWidget<'_> {
                 }
             }
             EventMsg::TaskStarted => {
+                self.bottom_pane.clear_ctrl_c_quit_hint();
                 self.bottom_pane.set_task_running(true);
                 self.request_redraw();
             }
@@ -402,6 +404,22 @@ impl ChatWidget<'_> {
         self.request_redraw();
     }
 
+    /// Handle Ctrl-C key press.
+    /// Returns true if the key press was handled, false if it was not.
+    /// If the key press was not handled, the caller should handle it (likely by exiting the process).
+    pub(crate) fn on_ctrl_c(&mut self) -> bool {
+        if self.bottom_pane.is_task_running() {
+            self.bottom_pane.clear_ctrl_c_quit_hint();
+            self.submit_op(Op::Interrupt);
+            false
+        } else if self.bottom_pane.ctrl_c_quit_hint_visible() {
+            true
+        } else {
+            self.bottom_pane.show_ctrl_c_quit_hint();
+            false
+        }
+    }
+
     /// Forward an `Op` directly to codex.
     pub(crate) fn submit_op(&self, op: Op) {
         if let Err(e) = self.codex_op_tx.send(op) {