Turn a web application into an installable offline PWA

We already have a working web application and want it to behave like a full-fledged mobile application, but without rewriting everything from scratch. The task is to turn the current frontend into a Progressive Web App so that visitors from smartphones and tablets can install it on the home screen, launch it in a separate window without the browser address bar and continue using key screens even when offline. Backend and business logic remain as is; work is focused on the client side and the offline access layer around it.

Specifically needed is a service worker that caches the application shell and critical resources, as well as a reasonable API response caching strategy so that previously loaded data remains available offline and synchronizes back when the network is restored. A correct web app manifest with icons, theme colors and proper startup URL should make the application installable on Android and iOS, and I want the install prompt handled neatly rather than left to its own devices. On top of that, you need to fully configure push notifications: request permission, store the subscription and a simple way to send a test push.

Performance and reliability are part of the result: the app should pass installability audit, degrade properly with an offline stub page and not break existing behavior for desktop users.

— Service worker with caching of shell and resources
— Offline stub page and API read cache
— Web app manifest, icons and install handling
— Push notifications: permission, subscription, test sending
— Passing installability audit/Lighthouse PWA
— Documentation for deploy and cache versioning