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/onboarding/trust_directory.rs

@@ -1,8 +1,6 @@
 use std::path::PathBuf;
 
 use codex_core::config::set_project_trusted;
-use codex_core::protocol::AskForApproval;
-use codex_core::protocol::SandboxPolicy;
 use crossterm::event::KeyCode;
 use crossterm::event::KeyEvent;
 use ratatui::buffer::Buffer;
@@ -22,9 +20,6 @@ use crate::onboarding::onboarding_screen::KeyboardHandler;
 use crate::onboarding::onboarding_screen::StepStateProvider;
 
 use super::onboarding_screen::StepState;
-use crate::app::ChatWidgetArgs;
-use std::sync::Arc;
-use std::sync::Mutex;
 
 pub(crate) struct TrustDirectoryWidget {
     pub codex_home: PathBuf,
@@ -33,11 +28,10 @@ pub(crate) struct TrustDirectoryWidget {
     pub selection: Option<TrustDirectorySelection>,
     pub highlighted: TrustDirectorySelection,
     pub error: Option<String>,
-    pub chat_widget_args: Arc<Mutex<ChatWidgetArgs>>,
 }
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
-pub(crate) enum TrustDirectorySelection {
+pub enum TrustDirectorySelection {
     Trust,
     DontTrust,
 }
@@ -156,13 +150,6 @@ impl TrustDirectoryWidget {
             // self.error = Some("Failed to set project trusted".to_string());
         }
 
-        // Update the in-memory chat config for this session to a more permissive
-        // policy suitable for a trusted workspace.
-        if let Ok(mut args) = self.chat_widget_args.lock() {
-            args.config.approval_policy = AskForApproval::OnRequest;
-            args.config.sandbox_policy = SandboxPolicy::new_workspace_write_policy();
-        }
-
         self.selection = Some(TrustDirectorySelection::Trust);
     }