Align frontend device types with backend canonical list

Replaces the old frontend-only `router` type and adds the full set used
by the backend: network_appliance, home_security, home_appliance, watch,
pc, gaming_console. Adds an `apiName` getter to map camelCase enum values
to the snake_case strings the API expects. Updates the unknown device icon
to a question mark.

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

frontend/lib/devices/device_actions.dart

@@ -121,7 +121,7 @@ Future<void> showRegisterDeviceDialog(
     await BackendAPI.instance.registerDevice(
       device.macAddress,
       owner,
-      deviceType.name,
+      deviceType.apiName,
     );
     if (!context.mounted) return;
     UISnackbars.showSuccess(context, 'Device registered');