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/bottom_pane/chat_composer.rs

@@ -0,0 +1,117 @@
+use crossterm::event::KeyEvent;
+use ratatui::buffer::Buffer;
+use ratatui::layout::Alignment;
+use ratatui::layout::Rect;
+use ratatui::style::Style;
+use ratatui::style::Stylize;
+use ratatui::text::Line;
+use ratatui::widgets::BorderType;
+use ratatui::widgets::Borders;
+use ratatui::widgets::Widget;
+use ratatui::widgets::WidgetRef;
+use tui_textarea::Input;
+use tui_textarea::Key;
+use tui_textarea::TextArea;
+
+/// Minimum number of visible text rows inside the textarea.
+const MIN_TEXTAREA_ROWS: usize = 1;
+/// Rows consumed by the border.
+const BORDER_LINES: u16 = 2;
+
+/// Result returned when the user interacts with the text area.
+pub enum InputResult {
+    Submitted(String),
+    None,
+}
+
+pub(crate) struct ChatComposer<'a> {
+    textarea: TextArea<'a>,
+}
+
+impl ChatComposer<'_> {
+    pub fn new(has_input_focus: bool) -> Self {
+        let mut textarea = TextArea::default();
+        textarea.set_placeholder_text("send a message");
+        textarea.set_cursor_line_style(ratatui::style::Style::default());
+
+        let mut this = Self { textarea };
+        this.update_border(has_input_focus);
+        this
+    }
+
+    pub fn set_input_focus(&mut self, has_focus: bool) {
+        self.update_border(has_focus);
+    }
+
+    /// Handle key event when no overlay is present.
+    pub fn handle_key_event(&mut self, key_event: KeyEvent) -> (InputResult, bool) {
+        match key_event.into() {
+            Input {
+                key: Key::Enter,
+                shift: false,
+                alt: false,
+                ctrl: false,
+            } => {
+                let text = self.textarea.lines().join("\n");
+                self.textarea.select_all();
+                self.textarea.cut();
+                (InputResult::Submitted(text), true)
+            }
+            Input {
+                key: Key::Enter, ..
+            }
+            | Input {
+                key: Key::Char('j'),
+                ctrl: true,
+                alt: false,
+                shift: false,
+            } => {
+                self.textarea.insert_newline();
+                (InputResult::None, true)
+            }
+            input => {
+                self.textarea.input(input);
+                (InputResult::None, true)
+            }
+        }
+    }
+
+    pub fn calculate_required_height(&self, _area: &Rect) -> u16 {
+        let rows = self.textarea.lines().len().max(MIN_TEXTAREA_ROWS);
+        rows as u16 + BORDER_LINES
+    }
+
+    fn update_border(&mut self, has_focus: bool) {
+        struct BlockState {
+            right_title: Line<'static>,
+            border_style: Style,
+        }
+
+        let bs = if has_focus {
+            BlockState {
+                right_title: Line::from("Enter to send | Ctrl+D to quit | Ctrl+J for newline")
+                    .alignment(Alignment::Right),
+                border_style: Style::default(),
+            }
+        } else {
+            BlockState {
+                right_title: Line::from(""),
+                border_style: Style::default().dim(),
+            }
+        };
+
+        self.textarea.set_block(
+            ratatui::widgets::Block::default()
+                .title_bottom(bs.right_title)
+                .borders(Borders::ALL)
+                .border_type(BorderType::Rounded)
+                .border_style(bs.border_style),
+        );
+    }
+}
+
+impl WidgetRef for &ChatComposer<'_> {
+    fn render_ref(&self, area: Rect, buf: &mut Buffer) {
+        self.textarea.render(area, buf);
+    }
+}