How reminders work (please read): This is a
dependency-free web app with no server, so it cannot do true
background push. Reminders fire reliably only while the app (or its
service worker) is awake — typically when the app is open or was used
recently. Android may also delay or drop notifications when the app
has been closed for a long time or the battery saver is on. For a
guaranteed alarm, also set a normal phone alarm/calendar event. The
code documents this honestly; it does not pretend to be a push
service.