reactimer

This version adds the Audio background mode. The app signals the start and end of each round, plus randomized mid-round cues, with audio bells. Users run it with the phone pocketed or the screen locked during training, so these bells must sound while the app is in the background — a round timer that goes silent when the screen locks does not do its job. To verify: on the main timer screen, configure a short round (e.g. 30 seconds), press Start, then lock the screen or switch to another app. The bell sounds at the end of the round and at each signal cue while the app is backgrounded.