feat(ui): context- and state-aware bottom keybinding strip (#27)

Per ADR-0051 (closing issue #27): the bottom status line is now a
keystrokes-only, state-selected strip. A pure status_bar_bindings()
picks the binding set by priority (first match wins):

  sidebar focus   → Ctrl-O next pane · ↑↓/PgUp/PgDn scroll · Esc input
  completion live → Tab/Shift-Tab cycle · Esc cancel · Enter run
  history nav     → ↑↓ browse · Esc clear · Enter run
  editing         → Esc clear · Ctrl-A/E home/end · Ctrl-W del word · Enter run
  default (empty) → Ctrl-O sidebar · Tab complete · ↑ history · Enter run

The editing state surfaces the #29 readline keys (ADR-0049's deferred
advertisement). Typed-command words (mode advanced/simple, the ':'
one-shot) and Ctrl-C quit leave the strip; simple mode's empty-input
hint gains a '`mode advanced` for SQL' pointer (advanced mode shows
none — a switcher knows the way back, and help covers it).

Mechanism: status_bar_bindings + a thin renderer (unit-testable);
App::is_browsing_history() exposes the private history_cursor; the mode
pointer lives in resolve_hint_lines. Catalog: 12 new shortcut labels +
panel.hint_mode_advanced (en-US.yaml + keys.rs, validator 1:1), 5 dead
strip strings removed.

Forks user-chosen: editing state shows #29 keys; quit omitted; no
width-drop machinery (longest strip ~65 cols fits; a width-budget test
keeps it lean). Modal-aware strip is OOS (pre-existing). Tests: 9
Tier-1 unit (per-state key sets, width budget, mode pointer), 1 Tier-3
rewritten, 15 full-panel snapshots re-accepted (reviewed). 2467 pass /
0 fail / 0 skip, clippy clean.

src/app.rs

@@ -646,6 +646,17 @@ impl App {
         }
     }
 
+    /// Whether the user is currently browsing a recalled history entry
+    /// (Up/Down navigation, unedited). Exposes the private
+    /// `history_cursor` predicate so the context-aware status strip
+    /// (ADR-0051) can select its history-navigation state. Editing the
+    /// recalled line ends navigation (`cancel_history_navigation`), so
+    /// this is `false` again the moment the user types.
+    #[must_use]
+    pub const fn is_browsing_history(&self) -> bool {
+        self.history_cursor.is_some()
+    }
+
     /// The input view the **live-feedback** walkers (completion, ambient
     /// hint, validity verdict, highlight overlays) should see, plus the
     /// byte offset stripped from the front and the cursor mapped into the