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/insert_history.rs

@@ -22,7 +22,7 @@ use textwrap::Options as TwOptions;
 use textwrap::WordSplitter;
 
 /// Insert `lines` above the viewport.
-pub(crate) fn insert_history_lines(terminal: &mut tui::Tui, lines: Vec<Line>) {
+pub(crate) fn insert_history_lines(terminal: &mut tui::Terminal, lines: Vec<Line>) {
     let mut out = std::io::stdout();
     insert_history_lines_to_writer(terminal, &mut out, lines);
 }
@@ -39,7 +39,7 @@ pub fn insert_history_lines_to_writer<B, W>(
 {
     let screen_size = terminal.backend().size().unwrap_or(Size::new(0, 0));
 
-    let mut area = terminal.get_frame().area();
+    let mut area = terminal.viewport_area;
 
     // Pre-wrap lines using word-aware wrapping so terminal scrollback sees the same
     // formatting as the TUI. This avoids character-level hard wrapping by the terminal.