How would you design a PWA with reliable offline support, app installability, and push notifications — covering the service worker lifecycle and iOS/Android platform differences?

Imagine you have a magic coloring book that works even when you lose your crayons, and it can tap you on the shoulder when it's time to color. That's what a PWA does: it saves your app so it works without internet, can be added to your phone's home screen like a real app, and can send you reminders even when the app is closed.