fix(tui): address Copilot round-3 review on #19835

Three classes of robustness issue caught on the second pass — all
revolve around malformed YAML tipping ``parseVoiceRecordKey`` or
``_voice_record_key`` into a crash instead of the documented
fallback.

* **Parser crashed on non-string YAML scalars.** ``config.get full``
  returns raw ``yaml.safe_load`` output, so ``voice.record_key: 1``
  or ``voice.record_key: true`` in a hand-edited config would hit
  ``.trim()`` on a number/bool and throw, breaking startup and
  every mtime re-apply. Accept ``unknown`` at the signature, guard
  with ``typeof raw !== 'string'``, and fall back to the default.

* **Backend blew up on non-dict ``voice:``.** Same YAML hazard on
  the gateway side: ``voice: true`` / ``voice: cmd+b`` left
  ``_load_cfg().get("voice")`` as a bool/str, so ``.get("record_key")``
  raised AttributeError and took every ``voice.toggle`` branch down
  with it. Centralised the lookup in a single
  ``_voice_record_key()`` helper that ``isinstance``-guards both
  ``voice`` and ``record_key`` and falls back to ``ctrl+b``.

* **Multi-modifier chords silently dropped extras.** The previous
  validator only checked the first modifier token, so ``ctrl+alt+r``
  silently parsed as ``ctrl+r`` and ``cmd+ctrl+b`` as ``super+b`` —
  a typo bound a different shortcut than the user configured.
  Reject multi-modifier spellings outright; the classic CLI only
  supports single-modifier bindings via prompt_toolkit's ``c-x`` /
  ``a-x`` rewrite, so this matches CLI parity.

Coverage added:

* ``parseVoiceRecordKey`` fallback on ``1`` / ``true`` / ``null`` /
  ``undefined`` / ``{}``.
* ``parseVoiceRecordKey`` fallback on ``ctrl+alt+r`` /
  ``cmd+ctrl+b`` / ``alt+ctrl+space``.
* ``test_voice_toggle_handles_non_dict_voice_cfg`` exercises
  every non-dict ``voice:`` shape (bool, str, None, int, list) and
  asserts each falls back to ``record_key: 'ctrl+b'``.

Suite: 581/581 TUI vitest green, 3/3 backend voice tests green,
tsc --noEmit clean.

tests/test_tui_gateway_server.py

@@ -106,6 +106,35 @@ def test_voice_toggle_returns_configured_record_key(monkeypatch):
     assert status_resp["result"]["record_key"] == "ctrl+o"
 
 
+def test_voice_toggle_handles_non_dict_voice_cfg(monkeypatch):
+    """Round-3 Copilot review regression on #19835.
+
+    ``_load_cfg()`` is raw ``yaml.safe_load()`` output — a hand-edited
+    ``voice: true`` / ``voice: cmd+b`` / ``voice: null`` leaves ``voice``
+    as a bool/str/None, not a dict. Previously ``.get("record_key")``
+    on a non-dict broke every ``voice.toggle`` branch. Now it falls
+    back to the documented default.
+    """
+    monkeypatch.setitem(
+        sys.modules,
+        "tools.voice_mode",
+        types.SimpleNamespace(
+            check_voice_requirements=lambda: {"available": True, "details": ""}
+        ),
+    )
+
+    for bad in (True, "cmd+b", None, 42, ["ctrl+b"]):
+        monkeypatch.setattr(server, "_load_cfg", lambda b=bad: {"voice": b})
+
+        status_resp = server.dispatch(
+            {"id": "voice-status", "method": "voice.toggle", "params": {"action": "status"}}
+        )
+
+        assert status_resp["result"]["record_key"] == "ctrl+b", (
+            f"voice.record_key fell back to default for voice={bad!r}"
+        )
+
+
 def test_voice_toggle_tts_branch_also_carries_record_key(monkeypatch):
     """Round-2 Copilot review regression on #19835.