Make this one more vote for NOT keeping the clutch depressed when waiting at traffic lights.
While this practice is not likely to lead to problems with the brake hydraulic system, there are other factors to consider. In addition to putting additional stress on the clutch throw-out bearing, I can give you another reason for not doing what you have been doing! While you are sitting there with the clutch depressed, if another vehicle happens to “tap” you from the rear, there is a real chance that your foot will slip off of the clutch, and that might cause you to hit someone else waiting in front of you.
With two valid reasons for not keeping the clutch pedal depressed at traffic lights, I am confident that you will modify your habits.