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/friendly/strings/en-US.yaml

@@ -883,14 +883,21 @@ panel:
   relationships_title: "Relationships"
   relationships_empty: "(none)"
   hint_empty: "Type a command — press Tab for options, `help` for a list"
+  # Mode-discovery pointer appended to the empty-input hint in SIMPLE
+  # mode (ADR-0051): the `mode advanced` switch left the keybinding
+  # strip, so the hint advertises it. Leading separator continues the
+  # prompt line. Advanced mode shows no pointer — users know how they
+  # got there, and `help` covers the way back.
+  hint_mode_advanced: " · `mode advanced` for SQL"
   # Panel titles for the output and hint panels (rendered inside
   # the rounded border, hence the leading/trailing space).
   output_title: "Output"
   hint_title: "Hint"
 
 # ---- Shortcut hints (paired with key names in the bottom bar) -------
+# The bottom strip is keystrokes-only and state-aware (ADR-0051). Labels
+# pair with a key name in the renderer (e.g. `Enter` + `run`).
 shortcut:
-  submit: "submit"
   confirm: "confirm"
   cancel: "cancel"
   yes: "Yes"
@@ -899,10 +906,19 @@ shortcut:
   select: "select"
   browse_path: "browse path"
   back_to_list: "back to list"
-  switch: "switch"
-  advanced_once: "advanced once"
-  cancel_one_shot: "cancel one-shot"
-  quit: "quit"
+  # Status-strip labels (ADR-0051, issue #27).
+  run: "run"
+  nav: "sidebar"
+  next_pane: "next pane"
+  scroll: "scroll"
+  to_input: "input"
+  cycle: "cycle"
+  browse: "browse"
+  clear: "clear"
+  complete: "complete"
+  history: "history"
+  home_end: "home/end"
+  del_word: "del word"
 
 # ---- mode / messages banners (app-level commands) -------------------
 mode: