fix(tui): address Copilot round-2 review on #19835

Three tightenings on the surface introduced in the round-1 fix:

* **``/voice tts`` reset custom bindings to Ctrl+B.** The ``tts`` branch
  of ``voice.toggle`` omitted ``record_key`` from its response, so the
  frontend's ``r.record_key ?? 'ctrl+b'`` coerced a user's custom
  binding back to the default on every TTS toggle. Two-sided fix:
  the backend now includes ``record_key`` on the ``tts`` branch (parity
  with ``status``/``on``/``off``), and the slash handler only pushes
  frontend state when the response actually carries ``record_key`` —
  belt-and-suspenders against any future branch forgetting to include
  it.

* **``super+b`` / ``win+b`` / ``cmd+b`` displayed "Cmd+B" on Linux and
  Windows.** ``formatVoiceRecordKey`` rendered ``mod === 'super'`` as
  ``Cmd`` universally, which told non-mac users the wrong modifier to
  press even though ``isVoiceToggleKey`` matched the right event bits.
  Gate the label to ``isMac`` so non-mac renders ``Super+B``.

* **``control+b`` / ``ctrl + b`` lost the macOS Cmd+B fallback.**
  ``_isDefaultVoiceKey`` keyed off ``parsed.raw`` — so
  semantically-equal aliases of the documented default dropped into
  the strict branch even though they bind Ctrl+B. Compare on the
  parsed spec (mod + ch + named) instead.

Coverage added: Linux ``Super+B`` rendering (and macOS ``Cmd+B``),
``control+b`` / ``ctrl + b`` accepting the Cmd+B fallback on darwin,
``/voice tts`` without ``record_key`` not clobbering cached binding,
and a backend regression asserting every ``voice.toggle`` branch
carries the configured key.

Suite: 579/579 TUI vitest green, 2/2 backend voice tests green,
tsc --noEmit clean.

tests/test_tui_gateway_server.py

@@ -106,6 +106,38 @@ def test_voice_toggle_returns_configured_record_key(monkeypatch):
     assert status_resp["result"]["record_key"] == "ctrl+o"
 
 
+def test_voice_toggle_tts_branch_also_carries_record_key(monkeypatch):
+    """Round-2 Copilot review regression on #19835.
+
+    The ``tts`` branch used to omit ``record_key`` from its response, so a
+    TUI client would parse ``r.record_key ?? 'ctrl+b'`` and reset a
+    custom binding to the default on every TTS toggle. Every branch of
+    ``voice.toggle`` now carries the configured key so frontend state
+    stays authoritative.
+    """
+    monkeypatch.setattr(
+        server,
+        "_load_cfg",
+        lambda: {"voice": {"record_key": "ctrl+space"}},
+    )
+    monkeypatch.setitem(
+        sys.modules,
+        "tools.voice_mode",
+        types.SimpleNamespace(
+            check_voice_requirements=lambda: {"available": True, "details": ""}
+        ),
+    )
+    monkeypatch.setenv("HERMES_VOICE", "1")
+    monkeypatch.delenv("HERMES_VOICE_TTS", raising=False)
+
+    tts_resp = server.dispatch(
+        {"id": "voice-tts", "method": "voice.toggle", "params": {"action": "tts"}}
+    )
+
+    assert tts_resp["result"]["record_key"] == "ctrl+space"
+    assert tts_resp["result"]["tts"] is True
+
+
 def test_load_enabled_toolsets_prefers_tui_env(monkeypatch):
     monkeypatch.setenv("HERMES_TUI_TOOLSETS", "web, terminal, ,memory")