feat(ui): Ctrl-O navigation mode — peek + expand the schema sidebar (#21, ADR-0046 DC1/DC2/DC4)

Ctrl-O enters a navigation mode orthogonal to the input mode, cycling
focus Input -> Tables -> Relationships -> Input (Esc exits). While a
sidebar panel is focused the sidebar is revealed (a peek, even when
width-hidden) and drawn as an expanded 45-column overlay over a cleared
main area, so the schema is browsable without the cramped 26-column
unfocused width. The focused panel gets an accent border.

Routing lives in the main key handler after the modal gate, so Ctrl-O
and nav keys are inert while a modal is open; in nav mode every
non-navigation key (printable/Enter/Tab/Backspace/...) is inert because
the input is occluded. Scroll keys (Up/Down, PageUp/PageDown) are
reserved for DC3 (next).

New App state: NavFocus { Input, SidebarTables, SidebarRelationships }.
Tests: the focus cycle, Esc exit, input-keys-inert, overlay reveal +
expansion, the accent-border style, and an overlay snapshot.

src/ui.rs

@@ -12,7 +12,9 @@ use ratatui::style::{Modifier, Style};
 use ratatui::text::{Line, Span};
 use ratatui::widgets::{Block, BorderType, Borders, Paragraph, Wrap};
 
-use crate::app::{App, EchoStatus, EffectiveMode, OutputKind, OutputLine, OutputStyleClass};
+use crate::app::{
+    App, EchoStatus, EffectiveMode, NavFocus, OutputKind, OutputLine, OutputStyleClass,
+};
 use crate::mode::Mode;
 use crate::theme::Theme;
 
@@ -96,6 +98,15 @@ pub fn render(app: &mut App, theme: &Theme, frame: &mut Frame<'_>) {
     render_project_label(app, theme, frame, outer[1]);
     render_status_bar(app, theme, frame, outer[2]);
 
+    // ADR-0046 DC2: in navigation mode, draw the focused sidebar as an
+    // expanded overlay over the (unchanged) base render — revealing it
+    // if it was hidden (peek) and widening it for browsing. Drawn below
+    // the modal layer; a modal can't open in navigation mode, but if one
+    // is somehow up it still wins.
+    if app.nav_focus.in_sidebar() {
+        render_nav_sidebar_overlay(app, theme, frame, outer[0]);
+    }
+
     // Modal dialogs (rebuild confirm, save-as prompt, load
     // picker, …) are drawn last so they overlay the rest of
     // the frame.
@@ -104,6 +115,54 @@ pub fn render(app: &mut App, theme: &Theme, frame: &mut Frame<'_>) {
     }
 }
 
+/// Width (columns) of the navigation-mode expanded sidebar overlay
+/// (ADR-0046 DC2). Wide enough that most relationship endpoints fit on
+/// one line, turning horizontal truncation into vertical scrolling.
+const NAV_EXPANDED_WIDTH: u16 = 45;
+
+/// Draw the focused sidebar, expanded, as an overlay over the left of
+/// the main content area (ADR-0046 DC2/DC3). `Clear` + a background
+/// repaint hide the base render underneath; the two panels keep the
+/// DB4 split. The focused panel is accent-bordered (DC3).
+fn render_nav_sidebar_overlay(app: &App, theme: &Theme, frame: &mut Frame<'_>, area: Rect) {
+    // Clear the whole main content region (the "affected right-column
+    // region", DC2) and repaint the background, so the base output /
+    // input / hint do not show through half-occluded. They are restored
+    // unchanged on the next frame when navigation mode exits.
+    frame.render_widget(ratatui::widgets::Clear, area);
+    paint_background(theme, frame, area);
+
+    // Paint the expanded sidebar over the left; the rest stays blank
+    // background while browsing.
+    let width = NAV_EXPANDED_WIDTH.min(area.width);
+    let sidebar = Rect {
+        x: area.x,
+        y: area.y,
+        width,
+        height: area.height,
+    };
+    let rel_content = (app.relationships.len() as u16).saturating_mul(3);
+    let rel_h = relationships_panel_height(sidebar.height, rel_content);
+    let parts = Layout::default()
+        .direction(Direction::Vertical)
+        .constraints([Constraint::Min(3), Constraint::Length(rel_h)])
+        .split(sidebar);
+    render_items_panel(app, theme, frame, parts[0]);
+    render_relationships_panel(app, theme, frame, parts[1]);
+}
+
+/// Border style for a sidebar panel: an accented, bold border when it
+/// holds navigation focus (ADR-0046 DC3), the muted border otherwise.
+fn panel_border_style(theme: &Theme, focused: bool) -> Style {
+    if focused {
+        Style::default()
+            .fg(theme.fg)
+            .add_modifier(Modifier::BOLD)
+    } else {
+        Style::default().fg(theme.border)
+    }
+}
+
 fn render_modal(modal: &crate::app::Modal, theme: &Theme, frame: &mut Frame<'_>, area: Rect) {
     use crate::app::Modal;
     match modal {
@@ -684,7 +743,10 @@ fn render_items_panel(app: &App, theme: &Theme, frame: &mut Frame<'_>, area: Rec
     let block = Block::default()
         .borders(Borders::ALL)
         .border_type(BorderType::Rounded)
-        .border_style(Style::default().fg(theme.border))
+        .border_style(panel_border_style(
+            theme,
+            app.nav_focus == NavFocus::SidebarTables,
+        ))
         .title(Span::styled(
             format!(" {} ", crate::t!("panel.tables_title")),
             Style::default()
@@ -747,7 +809,10 @@ fn render_relationships_panel(app: &App, theme: &Theme, frame: &mut Frame<'_>, a
     let block = Block::default()
         .borders(Borders::ALL)
         .border_type(BorderType::Rounded)
-        .border_style(Style::default().fg(theme.border))
+        .border_style(panel_border_style(
+            theme,
+            app.nav_focus == NavFocus::SidebarRelationships,
+        ))
         .title(Span::styled(
             format!(" {} ", crate::t!("panel.relationships_title")),
             Style::default()
@@ -2788,4 +2853,57 @@ mod tests {
         let snapshot = render_to_string(&mut app, &theme, 110, 24);
         insta::assert_snapshot!("relationships_panel_dark", snapshot);
     }
+
+    #[test]
+    fn navigation_mode_reveals_and_expands_the_sidebar() {
+        // ADR-0046 DC1/DC2: at a narrow width the sidebar is hidden, but
+        // focusing a sidebar panel peeks it open as an expanded overlay.
+        let mut app = App::new();
+        app.tables = vec!["Customers".to_string()];
+        app.relationships = vec![one_relationship()];
+        let theme = Theme::dark();
+        let normal = render_to_string(&mut app, &theme, 80, 24);
+        assert!(
+            !normal.contains("Tables"),
+            "sidebar hidden at 80 wide when not browsing:\n{normal}"
+        );
+
+        app.nav_focus = NavFocus::SidebarTables;
+        let focused = render_to_string(&mut app, &theme, 80, 24);
+        assert!(focused.contains("Tables"), "sidebar revealed in nav mode:\n{focused}");
+        assert!(focused.contains("Customers"), "tables in the overlay:\n{focused}");
+        assert!(
+            focused.contains("Relationships"),
+            "relationships panel in the overlay:\n{focused}"
+        );
+        assert!(
+            focused.contains("Customers_Orders"),
+            "relationship listed in the overlay:\n{focused}"
+        );
+    }
+
+    #[test]
+    fn focused_panel_gets_an_accent_border() {
+        // ADR-0046 DC3: the focused sidebar panel is accent-bordered.
+        let theme = Theme::dark();
+        let focused = panel_border_style(&theme, true);
+        let normal = panel_border_style(&theme, false);
+        assert_eq!(focused.fg, Some(theme.fg));
+        assert!(focused.add_modifier.contains(Modifier::BOLD));
+        assert_eq!(normal.fg, Some(theme.border));
+        assert!(!normal.add_modifier.contains(Modifier::BOLD));
+    }
+
+    #[test]
+    fn navigation_overlay_snapshot() {
+        // The expanded overlay over a full-width base (sidebar hidden at
+        // 80), with the Relationships panel focused (accent border).
+        let mut app = App::new();
+        app.tables = vec!["Customers".to_string(), "Orders".to_string()];
+        app.relationships = vec![one_relationship()];
+        app.nav_focus = NavFocus::SidebarRelationships;
+        let theme = Theme::dark();
+        let snapshot = render_to_string(&mut app, &theme, 80, 24);
+        insta::assert_snapshot!("nav_overlay_relationships_focused_dark", snapshot);
+    }
 }