feat(ui): add configurable scrollback lines and new‑message alert badge

Introduce `ui.scrollback_lines` (default 2000) to cap the number of chat lines kept in memory, with `0` disabling trimming. Implement automatic trimming of older lines, maintain a scroll offset, and show a “↓ New messages (press G)” badge when new messages arrive off‑screen. Update core UI settings, TUI rendering, chat app state, migrations, documentation, and changelog to reflect the new feature.

crates/owlen-tui/src/ui.rs

@@ -826,6 +826,15 @@ fn render_messages(frame: &mut Frame<'_>, area: Rect, app: &mut ChatApp) {
         lines.push(Line::from("No messages yet. Press 'i' to start typing."));
     }
 
+    let scrollback_limit = app.scrollback_limit();
+    if scrollback_limit != usize::MAX && lines.len() > scrollback_limit {
+        let removed = lines.len() - scrollback_limit;
+        lines = lines.into_iter().skip(removed).collect();
+        app.apply_chat_scrollback_trim(removed, lines.len());
+    } else {
+        app.apply_chat_scrollback_trim(0, lines.len());
+    }
+
     // Apply visual selection highlighting if in visual mode and Chat panel is focused
     if matches!(app.mode(), InputMode::Visual)
         && matches!(app.focused_panel(), FocusedPanel::Chat)
@@ -860,6 +869,31 @@ fn render_messages(frame: &mut Frame<'_>, area: Rect, app: &mut ChatApp) {
 
     frame.render_widget(paragraph, area);
 
+    if app.has_new_message_alert() {
+        let badge_text = "↓ New messages (press G)";
+        let text_width = badge_text.chars().count() as u16;
+        let badge_width = text_width.saturating_add(2);
+        if area.width > badge_width + 1 && area.height > 2 {
+            let badge_x = area.x + area.width.saturating_sub(badge_width + 1);
+            let badge_y = area.y + 1;
+            let badge_area = Rect::new(badge_x, badge_y, badge_width, 1);
+            frame.render_widget(Clear, badge_area);
+            let badge_line = Line::from(Span::styled(
+                format!(" {badge_text} "),
+                Style::default()
+                    .fg(theme.background)
+                    .bg(theme.info)
+                    .add_modifier(Modifier::BOLD),
+            ));
+            frame.render_widget(
+                Paragraph::new(badge_line)
+                    .style(Style::default().bg(theme.info).fg(theme.background))
+                    .alignment(Alignment::Center),
+                badge_area,
+            );
+        }
+    }
+
     // Render cursor if Chat panel is focused and in Normal mode
     if matches!(app.focused_panel(), FocusedPanel::Chat) && matches!(app.mode(), InputMode::Normal)
     {