feat: expose scene presets as voice-controllable scene.* entities (0.5.0)

Add a SCENE platform (scene.py) so each scene preset becomes a scene.*
entity. Alexa / Google Assistant / HomeKit expose the scene, light, and
switch domains — but NOT button — so the existing button.* scene entities
are invisible to voice assistants. Scenes use a distinct unique_id so both
coexist (buttons for dashboards, scenes for voice).

- Register Platform.SCENE; bump manifest 0.4.0 -> 0.5.0
- README: "Voice Control (Alexa / Google / Siri)" guide (Nabu Casa cloud,
  manual skill linking, HomeKit bridge, voice-friendly naming)

custom_components/ledgrab/__init__.py

@@ -31,6 +31,7 @@ _LOGGER = logging.getLogger(__name__)
 PLATFORMS: list[Platform] = [
     Platform.BUTTON,
     Platform.LIGHT,
+    Platform.SCENE,
     Platform.SWITCH,
     Platform.SENSOR,
     Platform.NUMBER,