Add expand/collapse all, per-pane key legends

SectionTree:
- 'e': expand all nodes
- 'c': collapse all nodes
- Home/End already wired from previous commit

Key legend shown at bottom border of each focused pane:
- Tree panes: nav, expand/collapse, expand/collapse all, paging
- Agent list: select, tab
- History: scroll, paging

Legend only appears on the focused pane to avoid clutter.

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

src/user/widgets.rs

@@ -48,6 +48,14 @@ pub fn format_ts_age(ts: i64) -> String {
     format_age((now - ts).max(0) as f64)
 }
 
+/// Key legend for SectionTree panes.
+pub fn tree_legend() -> Line<'static> {
+    Line::styled(
+        " ↑↓:nav  →/Enter:expand  ←:collapse  e:expand all  c:collapse all  PgUp/Dn  Home/End ",
+        Style::default().fg(Color::DarkGray),
+    )
+}
+
 /// Render a paragraph with a vertical scrollbar.
 pub fn render_scrollable(
     frame: &mut Frame,
@@ -90,6 +98,14 @@ impl SectionTree {
         Self { selected: None, expanded: std::collections::HashSet::new(), scroll: 0 }
     }
 
+    /// Total nodes in the tree (regardless of expand state).
+    fn total_nodes(&self, sections: &[ContextSection]) -> usize {
+        fn count_all(section: &ContextSection) -> usize {
+            1 + section.children.iter().map(|c| count_all(c)).sum::<usize>()
+        }
+        sections.iter().map(|s| count_all(s)).sum()
+    }
+
     pub fn item_count(&self, sections: &[ContextSection]) -> usize {
         fn count(section: &ContextSection, expanded: &std::collections::HashSet<usize>, idx: &mut usize) -> usize {
             let my_idx = *idx;
@@ -146,6 +162,17 @@ impl SectionTree {
                     self.expanded.remove(&sel);
                 }
             }
+            KeyCode::Char('e') => {
+                // Expand all
+                let total = self.total_nodes(sections);
+                for i in 0..total {
+                    self.expanded.insert(i);
+                }
+            }
+            KeyCode::Char('c') => {
+                // Collapse all
+                self.expanded.clear();
+            }
             _ => {}
         }
         self.scroll_to_selected(height);