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.

ui-tui/src/__tests__/platform.test.ts

@@ -155,6 +155,30 @@ describe('parseVoiceRecordKey (#18994)', () => {
     expect(parseVoiceRecordKey('ctrl+spcae')).toEqual(DEFAULT_VOICE_RECORD_KEY)
     expect(parseVoiceRecordKey('ctrl+f5')).toEqual(DEFAULT_VOICE_RECORD_KEY)
   })
+
+  // Round-3 Copilot review regressions on #19835.
+  it('does not throw on non-string YAML scalars — falls back instead', async () => {
+    const { DEFAULT_VOICE_RECORD_KEY, parseVoiceRecordKey } = await importPlatform('linux')
+
+    // ``config.get full`` surfaces raw YAML values; ``voice.record_key: 1``
+    // or ``voice.record_key: true`` would otherwise crash ``.trim()``.
+    expect(parseVoiceRecordKey(1 as unknown as string)).toEqual(DEFAULT_VOICE_RECORD_KEY)
+    expect(parseVoiceRecordKey(true as unknown as string)).toEqual(DEFAULT_VOICE_RECORD_KEY)
+    expect(parseVoiceRecordKey(null as unknown as string)).toEqual(DEFAULT_VOICE_RECORD_KEY)
+    expect(parseVoiceRecordKey(undefined as unknown as string)).toEqual(DEFAULT_VOICE_RECORD_KEY)
+    expect(parseVoiceRecordKey({} as unknown as string)).toEqual(DEFAULT_VOICE_RECORD_KEY)
+  })
+
+  it('rejects multi-modifier chords rather than silently dropping extras', async () => {
+    const { DEFAULT_VOICE_RECORD_KEY, parseVoiceRecordKey } = await importPlatform('linux')
+
+    // Previously ``ctrl+alt+r`` parsed as ``ctrl+r`` and ``cmd+ctrl+b`` as
+    // ``super+b`` — a typo silently bound a different shortcut. Now a
+    // multi-modifier spelling falls back to the documented default.
+    expect(parseVoiceRecordKey('ctrl+alt+r')).toEqual(DEFAULT_VOICE_RECORD_KEY)
+    expect(parseVoiceRecordKey('cmd+ctrl+b')).toEqual(DEFAULT_VOICE_RECORD_KEY)
+    expect(parseVoiceRecordKey('alt+ctrl+space')).toEqual(DEFAULT_VOICE_RECORD_KEY)
+  })
 })
 
 describe('formatVoiceRecordKey (#18994)', () => {