feat: implement UI config flow with single-instance enforcement

custom_components/nestquest/__init__.py

@@ -19,9 +19,11 @@ async def async_setup(hass: HomeAssistant, config: dict[str, Any]) -> bool:
 async def async_setup_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
     """Set up NestQuest from a config entry."""
     hass.data.setdefault(DOMAIN, {})
+    hass.data[DOMAIN][entry.entry_id] = entry
     return True
 
 
 async def async_unload_entry(hass: HomeAssistant, entry: ConfigEntry) -> bool:
     """Unload a config entry."""
+    hass.data.setdefault(DOMAIN, {}).pop(entry.entry_id, None)
     return True