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/onboarding/mod.rs

@@ -1,5 +1,5 @@
 mod auth;
-mod continue_to_chat;
 pub mod onboarding_screen;
 mod trust_directory;
+pub use trust_directory::TrustDirectorySelection;
 mod welcome;