feat(ui): embed header in main block and base layout on inner content area

- Render the app title with version as the block title instead of a separate header widget.
- Compute `content_area` via `main_block.inner` and use it for file panel, main area, model info panel, and toast rendering.
- Remove header constraints and the `render_header` function, simplifying the layout.
- Add early exit when `content_area` has zero width or height to avoid rendering errors.

crates/owlen-tui/src/ui.rs

@@ -186,18 +186,37 @@ pub fn render_chat(frame: &mut Frame<'_>, app: &mut ChatApp) {
     // Set terminal background color
     let theme = app.theme().clone();
     let background_block = Block::default().style(Style::default().bg(theme.background));
-    let full_area = frame.area();
-    frame.render_widget(background_block, full_area);
+    let frame_area = frame.area();
+    frame.render_widget(background_block, frame_area);
 
-    let (file_area, main_area) = if app.is_file_panel_collapsed() || full_area.width < 40 {
-        (None, full_area)
+    let title_line = Line::from(vec![Span::styled(
+        format!(" 🦉 OWLEN v{} – AI Assistant ", APP_VERSION),
+        Style::default()
+            .fg(theme.focused_panel_border)
+            .add_modifier(Modifier::BOLD),
+    )]);
+    let main_block = Block::default()
+        .borders(Borders::ALL)
+        .border_style(Style::default().fg(theme.unfocused_panel_border))
+        .style(Style::default().bg(theme.background).fg(theme.text))
+        .title(title_line);
+
+    let content_area = main_block.inner(frame_area);
+    frame.render_widget(main_block, frame_area);
+
+    if content_area.width == 0 || content_area.height == 0 {
+        return;
+    }
+
+    let (file_area, main_area) = if app.is_file_panel_collapsed() || content_area.width < 40 {
+        (None, content_area)
     } else {
-        let max_sidebar = full_area.width.saturating_sub(30).max(10);
+        let max_sidebar = content_area.width.saturating_sub(30).max(10);
         let sidebar_width = app.file_panel_width().min(max_sidebar).max(10);
         let segments = Layout::default()
             .direction(Direction::Horizontal)
             .constraints([Constraint::Length(sidebar_width), Constraint::Min(30)])
-            .split(full_area);
+            .split(content_area);
         (Some(segments[0]), segments[1])
     };
 
@@ -253,10 +272,7 @@ pub fn render_chat(frame: &mut Frame<'_>, app: &mut ChatApp) {
         0
     };
 
-    let mut constraints = vec![
-        Constraint::Length(3), // Header
-        Constraint::Min(8),    // Messages
-    ];
+    let mut constraints = vec![Constraint::Min(8)]; // Messages
 
     if thinking_height > 0 {
         constraints.push(Constraint::Length(thinking_height)); // Thinking
@@ -276,9 +292,6 @@ pub fn render_chat(frame: &mut Frame<'_>, app: &mut ChatApp) {
         .split(chat_area);
 
     let mut idx = 0;
-    render_header(frame, layout[idx], app);
-    idx += 1;
-
     render_messages(frame, layout[idx], app);
     idx += 1;
 
@@ -318,13 +331,16 @@ pub fn render_chat(frame: &mut Frame<'_>, app: &mut ChatApp) {
     }
 
     if app.is_model_info_visible() {
-        let panel_width = full_area
+        let panel_width = content_area
             .width
             .saturating_div(3)
             .max(30)
-            .min(full_area.width.saturating_sub(20).max(30));
-        let x = full_area.x + full_area.width.saturating_sub(panel_width);
-        let area = Rect::new(x, full_area.y, panel_width, full_area.height);
+            .min(content_area.width.saturating_sub(20).max(30))
+            .min(content_area.width);
+        let x = content_area
+            .x
+            .saturating_add(content_area.width.saturating_sub(panel_width));
+        let area = Rect::new(x, content_area.y, panel_width, content_area.height);
         frame.render_widget(Clear, area);
         let viewport_height = area.height.saturating_sub(2) as usize;
         app.set_model_info_viewport_height(viewport_height);
@@ -335,7 +351,7 @@ pub fn render_chat(frame: &mut Frame<'_>, app: &mut ChatApp) {
         render_code_workspace(frame, area, app);
     }
 
-    render_toasts(frame, app, full_area);
+    render_toasts(frame, app, content_area);
 }
 
 fn toast_palette(level: ToastLevel, theme: &Theme) -> (&'static str, Style, Style) {
@@ -1109,24 +1125,6 @@ fn wrap_line_segments(line: &str, width: usize) -> Vec<String> {
     result
 }
 
-fn render_header(frame: &mut Frame<'_>, area: Rect, app: &ChatApp) {
-    let theme = app.theme();
-    let title_span = Span::styled(
-        format!(" 🦉 OWLEN v{} – AI Assistant ", APP_VERSION),
-        Style::default()
-            .fg(theme.focused_panel_border)
-            .add_modifier(Modifier::BOLD),
-    );
-
-    let header_block = Block::default()
-        .borders(Borders::ALL)
-        .border_style(Style::default().fg(theme.unfocused_panel_border))
-        .style(Style::default().bg(theme.background).fg(theme.text))
-        .title(Line::from(vec![title_span]));
-
-    frame.render_widget(header_block, area);
-}
-
 fn apply_visual_selection<'a>(
     lines: Vec<Line<'a>>,
     selection: Option<((usize, usize), (usize, usize))>,