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/Cargo.toml

@@ -22,6 +22,7 @@ workspace = true
 
 [dependencies]
 anyhow = "1"
+async-stream = "0.3.6"
 base64 = "0.22.1"
 chrono = { version = "0.4", features = ["serde"] }
 clap = { version = "4", features = ["derive"] }