Migrate GitWarning to OnboardingScreen (#1915)

This paves the way to do per-directory approval settings
(https://github.com/openai/codex/pull/1912).

This also lets us pass in a Config/ChatWidgetArgs into onboarding which
can then mutate it and emit the ChatWidgetArgs it wants at the end which
may be modified by the said approval settings.

<img width="1180" height="428" alt="CleanShot 2025-08-06 at 19 30 55"
src="https://github.com/user-attachments/assets/4dcfda42-0f5e-4b6d-a16d-2597109cc31c"
/>

codex-rs/tui/src/onboarding/welcome.rs

@@ -7,7 +7,13 @@ use ratatui::text::Line;
 use ratatui::text::Span;
 use ratatui::widgets::WidgetRef;
 
-pub(crate) struct WelcomeWidget {}
+use crate::onboarding::onboarding_screen::StepStateProvider;
+
+use super::onboarding_screen::StepState;
+
+pub(crate) struct WelcomeWidget {
+    pub is_logged_in: bool,
+}
 
 impl WidgetRef for &WelcomeWidget {
     fn render_ref(&self, area: Rect, buf: &mut Buffer) {
@@ -21,3 +27,12 @@ impl WidgetRef for &WelcomeWidget {
         line.render(area, buf);
     }
 }
+
+impl StepStateProvider for WelcomeWidget {
+    fn get_step_state(&self) -> StepState {
+        match self.is_logged_in {
+            true => StepState::Hidden,
+            false => StepState::Complete,
+        }
+    }
+}