user: share candidate-browser helpers between F6/F7

F6 (learn) and F7 (compare) were duplicating the candidate-screen
skeleton: outer magenta-bordered block with screen legend + title,
settings row / content / help vertical split, 40/60 list/detail
horizontal split, j/k/↑/↓ nav with bounds clamping.

Factor out three helpers in user/widgets.rs:

  candidate_frame(frame, area, title) -> (settings, content, help)
  list_detail_split(content) -> (list, detail)
  handle_list_nav(events, list_state, count, on_other)

Callers provide screen-specific content — settings line, empty state,
per-candidate list item, detail pane, help line, extra key bindings —
and the helpers absorb the common framing.

Net change is small in lines (-13 src) but removes the
copy-paste-and-tweak trap: F8/F9/whatever-next-screen now starts from
these three calls instead of a copy of learn.rs.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/user/widgets.rs

@@ -109,6 +109,73 @@ pub fn tree_legend() -> Line<'static> {
     )
 }
 
+// ---------------------------------------------------------------------------
+// Candidate-browser screen skeleton (F6 learn, F7 compare, future screens)
+// ---------------------------------------------------------------------------
+
+use ratatui::{
+    layout::{Constraint, Layout, Rect},
+    widgets::ListState,
+    crossterm::event::{Event, KeyEvent},
+    Frame,
+};
+
+/// Frame a candidate-browser screen: outer magenta-bordered block with
+/// the screen legend on the left and `title` on the right, split into
+/// (settings_row, content_area, help_row). Caller renders into the
+/// three sub-areas.
+pub fn candidate_frame(frame: &mut Frame, area: Rect, title: &str) -> (Rect, Rect, Rect) {
+    let block = Block::default()
+        .title_top(Line::from(super::screen_legend()).left_aligned())
+        .title_top(Line::from(format!(" {} ", title)).right_aligned())
+        .borders(Borders::ALL)
+        .border_style(Style::default().fg(Color::Magenta));
+    let inner = block.inner(area);
+    frame.render_widget(block, area);
+    let [settings, content] = Layout::vertical([
+        Constraint::Length(1), Constraint::Min(0),
+    ]).areas(inner);
+    let help = Rect { y: area.y + area.height - 1, height: 1, ..area };
+    (settings, content, help)
+}
+
+/// 40/60 horizontal split for list + detail panes within the content area.
+pub fn list_detail_split(content: Rect) -> (Rect, Rect) {
+    let [list, detail] = Layout::horizontal([
+        Constraint::Percentage(40), Constraint::Percentage(60),
+    ]).areas(content);
+    (list, detail)
+}
+
+/// Handle j/k/↑/↓ list navigation and keep the selection in bounds.
+/// Any other key is passed to `on_other` for screen-specific handling.
+pub fn handle_list_nav(
+    events: &[Event],
+    list_state: &mut ListState,
+    count: usize,
+    mut on_other: impl FnMut(KeyCode),
+) {
+    for event in events {
+        if let Event::Key(KeyEvent { code, .. }) = event {
+            match code {
+                KeyCode::Up | KeyCode::Char('k') => {
+                    let i = list_state.selected().unwrap_or(0);
+                    list_state.select(Some(i.saturating_sub(1)));
+                }
+                KeyCode::Down | KeyCode::Char('j') => {
+                    let i = list_state.selected().unwrap_or(0);
+                    list_state.select(Some((i + 1).min(count.saturating_sub(1))));
+                }
+                _ => on_other(*code),
+            }
+        }
+    }
+    if count > 0 {
+        let sel = list_state.selected().unwrap_or(0).min(count - 1);
+        list_state.select(Some(sel));
+    }
+}
+
 
 // ---------------------------------------------------------------------------
 // SectionTree — expand/collapse tree renderer for ContextSection