First pass at a TUI onboarding (#1876)

This sets up the scaffolding and basic flow for a TUI onboarding
experience. It covers sign in with ChatGPT, env auth, as well as some
safety guidance.

Next up:
1. Replace the git warning screen
2. Use this to configure default approval/sandbox modes


Note the shimmer flashes are from me slicing the video, not jank.

https://github.com/user-attachments/assets/0fbe3479-fdde-41f3-87fb-a7a83ab895b8

codex-rs/tui/src/shimmer.rs

@@ -0,0 +1,84 @@
+use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering;
+use std::time::Duration;
+
+use ratatui::style::Color;
+use ratatui::style::Modifier;
+use ratatui::style::Style;
+use ratatui::text::Span;
+
+use crate::app_event::AppEvent;
+use crate::app_event_sender::AppEventSender;
+
+#[derive(Debug)]
+pub(crate) struct FrameTicker {
+    running: Arc<AtomicBool>,
+}
+
+impl FrameTicker {
+    pub(crate) fn new(app_event_tx: AppEventSender) -> Self {
+        let running = Arc::new(AtomicBool::new(true));
+        let running_clone = running.clone();
+        let app_event_tx_clone = app_event_tx.clone();
+        std::thread::spawn(move || {
+            while running_clone.load(Ordering::Relaxed) {
+                std::thread::sleep(Duration::from_millis(100));
+                app_event_tx_clone.send(AppEvent::RequestRedraw);
+            }
+        });
+        Self { running }
+    }
+}
+
+impl Drop for FrameTicker {
+    fn drop(&mut self) {
+        self.running.store(false, Ordering::Relaxed);
+    }
+}
+
+pub(crate) fn shimmer_spans(text: &str, frame_idx: usize) -> Vec<Span<'static>> {
+    let chars: Vec<char> = text.chars().collect();
+    let padding = 10usize;
+    let period = chars.len() + padding * 2;
+    let pos = frame_idx % period;
+    let has_true_color = supports_color::on_cached(supports_color::Stream::Stdout)
+        .map(|level| level.has_16m)
+        .unwrap_or(false);
+    let band_half_width = 6.0;
+
+    let mut spans: Vec<Span<'static>> = Vec::with_capacity(chars.len());
+    for (i, ch) in chars.iter().enumerate() {
+        let i_pos = i as isize + padding as isize;
+        let pos = pos as isize;
+        let dist = (i_pos - pos).abs() as f32;
+
+        let t = if dist <= band_half_width {
+            let x = std::f32::consts::PI * (dist / band_half_width);
+            0.5 * (1.0 + x.cos())
+        } else {
+            0.0
+        };
+        let brightness = 0.4 + 0.6 * t;
+        let level = (brightness * 255.0).clamp(0.0, 255.0) as u8;
+        let style = if has_true_color {
+            Style::default()
+                .fg(Color::Rgb(level, level, level))
+                .add_modifier(Modifier::BOLD)
+        } else {
+            Style::default().fg(color_for_level(level))
+        };
+        spans.push(Span::styled(ch.to_string(), style));
+    }
+    spans
+}
+
+fn color_for_level(level: u8) -> Color {
+    if level < 128 {
+        Color::DarkGray
+    } else if level < 192 {
+        Color::Gray
+    } else {
+        Color::White
+    }
+}