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/app_event.rs

@@ -1,10 +1,7 @@
 use codex_core::protocol::Event;
 use codex_file_search::FileMatch;
-use crossterm::event::KeyEvent;
 use ratatui::text::Line;
-use std::time::Duration;
 
-use crate::app::ChatWidgetArgs;
 use crate::slash_command::SlashCommand;
 use codex_core::protocol::AskForApproval;
 use codex_core::protocol::SandboxPolicy;
@@ -15,21 +12,6 @@ use codex_core::protocol_config_types::ReasoningEffort;
 pub(crate) enum AppEvent {
     CodexEvent(Event),
 
-    /// Request a redraw which will be debounced by the [`App`].
-    RequestRedraw,
-
-    /// Actually draw the next frame.
-    Redraw,
-
-    /// Schedule a one-shot animation frame roughly after the given duration.
-    /// Multiple requests are coalesced by the central frame scheduler.
-    ScheduleFrameIn(Duration),
-
-    KeyEvent(KeyEvent),
-
-    /// Text pasted from the terminal clipboard.
-    Paste(String),
-
     /// Request to exit the application gracefully.
     ExitRequest,
 
@@ -63,10 +45,6 @@ pub(crate) enum AppEvent {
     StopCommitAnimation,
     CommitTick,
 
-    /// Onboarding: result of login_with_chatgpt.
-    OnboardingAuthComplete(Result<(), String>),
-    OnboardingComplete(ChatWidgetArgs),
-
     /// Update the current reasoning effort in the running app and widget.
     UpdateReasoningEffort(ReasoningEffort),