added "last checked" label to form

HalCheck/HalControle.cs

@@ -1885,6 +1885,10 @@ namespace HalCheck
             List<int> sensorIds = rthSensors.Select(s => s.Id).ToList();
             List<(double temperature, double humidity)> sensorData = GetTemperaturesAndHumidity(sensorIds);
 
+            lblLastCheckTimeStamp.BringToFront();
+            lblLastCheckTimeStamp.Visible = true;
+            lblLastCheckTimeStamp.Text = "Last checked: " + DateTime.Now.ToString("HH:mm:ss");
+
             for (int i = 0; i < sensorData.Count; i++)
             {
                 rthSensors[i].Temperature = sensorData[i].temperature;