resizable viewport (#1732)

Proof of concept for a resizable viewport.

The general approach here is to duplicate the `Terminal` struct from
ratatui, but with our own logic. This is a "light fork" in that we are
still using all the base ratatui functions (`Buffer`, `Widget` and so
on), but we're doing our own bookkeeping at the top level to determine
where to draw everything.

This approach could use improvement—e.g, when the window is resized to a
smaller size, if the UI wraps, we don't correctly clear out the
artifacts from wrapping. This is possible with a little work (i.e.
tracking what parts of our UI would have been wrapped), but this
behavior is at least at par with the existing behavior.


https://github.com/user-attachments/assets/4eb17689-09fd-4daa-8315-c7ebc654986d


cc @joshka who might have Thoughts™

codex-rs/tui/src/chatwidget.rs

@@ -143,6 +143,10 @@ impl ChatWidget<'_> {
         }
     }
 
+    pub fn desired_height(&self) -> u16 {
+        self.bottom_pane.desired_height()
+    }
+
     pub(crate) fn handle_key_event(&mut self, key_event: KeyEvent) {
         self.bottom_pane.clear_ctrl_c_quit_hint();