Application Kata - Exercise timer

Develop an application that can be used to time exercises during training sessions.

When an exercise begins, the trainer starts a timer with an exercise duration, e.g. 15 minutes. The exercise ends when the timer runs out - or when all participants have finished early. This means that each participant must be able to report to the application that they have already completed the exercise. (There should be a maximum delay of 30 seconds before the application receives such a message).

The application informs you of the end of the exercise for one reason or another by playing a sound (system sound or MP3/WAV file). It is sufficient for the trainer to be notified of the end of the exercise. The application also shows (at the latest at the end of the exercise) how many participants have finished early.

The application should not require trainers and/or participants to register.