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/Cargo.lock

@@ -926,6 +926,7 @@ name = "codex-tui"
 version = "0.0.0"
 dependencies = [
  "anyhow",
+ "async-stream",
  "base64 0.22.1",
  "chrono",
  "clap",