refactor onboarding screen to a separate "app" (#2524)

this is in preparation for adding more separate "modes" to the tui, in
particular, a "transcript mode" to view a full history once #2316 lands.

1. split apart "tui events" from "app events".
2. remove onboarding-related events from AppEvent.
3. move several general drawing tools out of App and into a new Tui
class

codex-rs/tui/src/bottom_pane/bottom_pane_view.rs

@@ -7,10 +7,10 @@ use super::BottomPane;
 use super::CancellationEvent;
 
 /// Trait implemented by every view that can be shown in the bottom pane.
-pub(crate) trait BottomPaneView<'a> {
+pub(crate) trait BottomPaneView {
     /// Handle a key event while the view is active. A redraw is always
     /// scheduled after this call.
-    fn handle_key_event(&mut self, _pane: &mut BottomPane<'a>, _key_event: KeyEvent) {}
+    fn handle_key_event(&mut self, _pane: &mut BottomPane, _key_event: KeyEvent) {}
 
     /// Return `true` if the view has finished and should be removed.
     fn is_complete(&self) -> bool {
@@ -18,7 +18,7 @@ pub(crate) trait BottomPaneView<'a> {
     }
 
     /// Handle Ctrl-C while this view is active.
-    fn on_ctrl_c(&mut self, _pane: &mut BottomPane<'a>) -> CancellationEvent {
+    fn on_ctrl_c(&mut self, _pane: &mut BottomPane) -> CancellationEvent {
         CancellationEvent::Ignored
     }