feat: add hotkey conflict detection and microphone listing helpers

Implementiere drei Hilfsfunktionen in _settings.py:
- check_hotkey_conflict(): Prüft mit Win32 RegisterHotKey ob eine Taste belegt ist
- list_microphones(): Gibt alle Eingabegeräte als (name, index) Tupel zurück
- pynput_to_evdev_key(): Konvertiert pynput Keys zu evdev Key-Namen

Alle Tests (9 neue + 9 existierende) bestehen.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_tray.py

@@ -105,3 +105,80 @@ class TestApplySystemTheme:
                 mock_set.assert_called_once_with("light")
         finally:
             root.destroy()
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Settings nur auf Windows")
+class TestCheckHotkeyConflict:
+    def test_returns_false_when_key_is_free(self):
+        from unittest.mock import patch, MagicMock
+        from whisper_local.tray._settings import check_hotkey_conflict
+
+        mock_user32 = MagicMock()
+        mock_user32.RegisterHotKey.return_value = 1  # Erfolg
+        with patch("ctypes.windll") as mock_windll:
+            mock_windll.user32 = mock_user32
+            result = check_hotkey_conflict("KEY_F12")
+        assert result is False
+        mock_user32.UnregisterHotKey.assert_called_once()
+
+    def test_returns_true_when_key_is_taken(self):
+        from unittest.mock import patch, MagicMock
+        from whisper_local.tray._settings import check_hotkey_conflict
+
+        mock_user32 = MagicMock()
+        mock_user32.RegisterHotKey.return_value = 0  # Belegt
+        with patch("ctypes.windll") as mock_windll:
+            mock_windll.user32 = mock_user32
+            result = check_hotkey_conflict("KEY_F12")
+        assert result is True
+        mock_user32.UnregisterHotKey.assert_not_called()
+
+    def test_returns_false_for_unknown_key(self):
+        from whisper_local.tray._settings import check_hotkey_conflict
+        result = check_hotkey_conflict("KEY_NONEXISTENT_999")
+        assert result is False
+
+
+class TestListMicrophones:
+    def test_returns_only_input_devices(self):
+        from unittest.mock import patch
+        from whisper_local.tray._settings import list_microphones
+
+        fake_devices = [
+            {"name": "Speakers", "max_input_channels": 0},
+            {"name": "Headset Mic", "max_input_channels": 1},
+            {"name": "USB Mic", "max_input_channels": 2},
+        ]
+        with patch("sounddevice.query_devices", return_value=fake_devices):
+            result = list_microphones()
+        assert result == [("Headset Mic", 1), ("USB Mic", 2)]
+
+    def test_returns_empty_list_when_no_input(self):
+        from unittest.mock import patch
+        from whisper_local.tray._settings import list_microphones
+
+        with patch("sounddevice.query_devices", return_value=[]):
+            result = list_microphones()
+        assert result == []
+
+
+class TestPynputToEvdevKey:
+    def test_function_key(self):
+        from pynput.keyboard import Key
+        from whisper_local.tray._settings import pynput_to_evdev_key
+        assert pynput_to_evdev_key(Key.f12) == "KEY_F12"
+
+    def test_space_key(self):
+        from pynput.keyboard import Key
+        from whisper_local.tray._settings import pynput_to_evdev_key
+        assert pynput_to_evdev_key(Key.space) == "KEY_SPACE"
+
+    def test_char_key(self):
+        from pynput.keyboard import KeyCode
+        from whisper_local.tray._settings import pynput_to_evdev_key
+        key = KeyCode.from_char("a")
+        assert pynput_to_evdev_key(key) == "KEY_A"
+
+    def test_unknown_returns_empty(self):
+        from whisper_local.tray._settings import pynput_to_evdev_key
+        assert pynput_to_evdev_key(None) == ""