chore: move each view used in BottomPane into its own file (#928)

`BottomPane` was getting a bit unwieldy because it maintained a
`PaneState` enum with three variants and many of its methods had `match`
statements to handle each variant. To replace the enum, this PR:

* Introduces a `trait BottomPaneView` that has two implementations:
`StatusIndicatorView` and `ApprovalModalView`.
* Migrates `PaneState::TextInput` into its own struct, `ChatComposer`,
that does **not** implement `BottomPaneView`.
* Updates `BottomPane` so it has `composer: ChatComposer` and
`active_view: Option<Box<dyn BottomPaneView<'a> + 'a>>`. The idea is
that `active_view` takes priority and is displayed when it is `Some`;
otherwise, `ChatComposer` is displayed.
* While methods of `BottomPane` often have to check whether
`active_view` is present to decide which component to delegate to, the
code is more straightforward than before and introducing new
implementations of `BottomPaneView` should be less painful.

Because we want to retain the `TextArea` owned by `ChatComposer` even
when another view is displayed, to keep the ownership logic simple, it
seemed best to keep `ChatComposer` distinct from `BottomPaneView`.

codex-rs/tui/src/chatwidget.rs

@@ -351,7 +351,7 @@ impl ChatWidget<'_> {
     pub(crate) fn update_latest_log(
         &mut self,
         line: String,
-    ) -> std::result::Result<(), std::sync::mpsc::SendError<AppEvent>> {
+    ) -> std::result::Result<(), SendError<AppEvent>> {
         // Forward only if we are currently showing the status indicator.
         self.bottom_pane.update_status_text(line)?;
         Ok(())
@@ -365,7 +365,7 @@ impl ChatWidget<'_> {
     pub(crate) fn handle_scroll_delta(
         &mut self,
         scroll_delta: i32,
-    ) -> std::result::Result<(), std::sync::mpsc::SendError<AppEvent>> {
+    ) -> std::result::Result<(), SendError<AppEvent>> {
         // If the user is trying to scroll exactly one line, we let them, but
         // otherwise we assume they are trying to scroll in larger increments.
         let magnified_scroll_delta = if scroll_delta == 1 {
@@ -389,7 +389,7 @@ impl ChatWidget<'_> {
 
 impl WidgetRef for &ChatWidget<'_> {
     fn render_ref(&self, area: Rect, buf: &mut Buffer) {
-        let bottom_height = self.bottom_pane.required_height(&area);
+        let bottom_height = self.bottom_pane.calculate_required_height(&area);
 
         let chunks = Layout::default()
             .direction(Direction::Vertical)