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/chatwidget/interrupts.rs

@@ -71,7 +71,7 @@ impl InterruptManager {
         self.queue.push_back(QueuedInterrupt::PatchEnd(ev));
     }
 
-    pub(crate) fn flush_all(&mut self, chat: &mut ChatWidget<'_>) {
+    pub(crate) fn flush_all(&mut self, chat: &mut ChatWidget) {
         while let Some(q) = self.queue.pop_front() {
             match q {
                 QueuedInterrupt::ExecApproval(id, ev) => chat.handle_exec_approval_now(id, ev),