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/bottom_pane/chat_composer.rs

@@ -38,6 +38,7 @@ pub(crate) struct ChatComposer<'a> {
     command_popup: Option<CommandPopup>,
     app_event_tx: AppEventSender,
     history: ChatComposerHistory,
+    ctrl_c_quit_hint: bool,
 }
 
 impl ChatComposer<'_> {
@@ -51,6 +52,7 @@ impl ChatComposer<'_> {
             command_popup: None,
             app_event_tx,
             history: ChatComposerHistory::new(),
+            ctrl_c_quit_hint: false,
         };
         this.update_border(has_input_focus);
         this
@@ -114,6 +116,11 @@ impl ChatComposer<'_> {
         self.update_border(has_focus);
     }
 
+    pub fn set_ctrl_c_quit_hint(&mut self, show: bool, has_focus: bool) {
+        self.ctrl_c_quit_hint = show;
+        self.update_border(has_focus);
+    }
+
     /// Handle a key event coming from the main UI.
     pub fn handle_key_event(&mut self, key_event: KeyEvent) -> (InputResult, bool) {
         let result = match self.command_popup {
@@ -304,10 +311,17 @@ impl ChatComposer<'_> {
         }
 
         let bs = if has_focus {
-            BlockState {
-                right_title: Line::from("Enter to send | Ctrl+D to quit | Ctrl+J for newline")
-                    .alignment(Alignment::Right),
-                border_style: Style::default(),
+            if self.ctrl_c_quit_hint {
+                BlockState {
+                    right_title: Line::from("Ctrl+C to quit").alignment(Alignment::Right),
+                    border_style: Style::default(),
+                }
+            } else {
+                BlockState {
+                    right_title: Line::from("Enter to send | Ctrl+D to quit | Ctrl+J for newline")
+                        .alignment(Alignment::Right),
+                    border_style: Style::default(),
+                }
             }
         } else {
             BlockState {