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/custom_terminal.rs

@@ -245,7 +245,7 @@ where
     /// Index of the current buffer in the previous array
     current: usize,
     /// Whether the cursor is currently hidden
-    hidden_cursor: bool,
+    pub hidden_cursor: bool,
     /// Area of the viewport
     pub viewport_area: Rect,
     /// Last known size of the terminal. Used to detect if the internal buffers have to be resized.