feat: add AppState enum and programmatic tray icon generation

Implements Task 5 of the TDD plan: creates the tray package with
AppState enum (WAITING, RECORDING, TRANSCRIBING) and a create_icon()
function that generates colorized microphone icons via Pillow.

- whisper_local/tray/__init__.py: empty package marker
- whisper_local/tray/_tray.py: AppState enum
- whisper_local/tray/_icon.py: icon generation with state-specific colors
- tests/test_tray.py: comprehensive test coverage for icon creation

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

tests/test_tray.py

@@ -0,0 +1,24 @@
+import sys
+import pytest
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="Tray nur auf Windows")
+class TestCreateIcon:
+    def test_returns_image_for_each_state(self):
+        from PIL import Image
+        from whisper_local.tray._tray import AppState
+        from whisper_local.tray._icon import create_icon
+
+        for state in AppState:
+            img = create_icon(state)
+            assert isinstance(img, Image.Image)
+            assert img.size == (64, 64)
+            assert img.mode == "RGBA"
+
+    def test_different_states_have_different_colors(self):
+        from whisper_local.tray._tray import AppState
+        from whisper_local.tray._icon import create_icon
+
+        waiting = create_icon(AppState.WAITING)
+        recording = create_icon(AppState.RECORDING)
+        assert waiting.tobytes() != recording.tobytes()