Test the Progressive Web App install flow (Web App Manifest validation, `beforeinstallprompt` event handling, installability criteria, install prompt UX). Covers desktop install badge, Android WebAPK minting, iOS Add to Home Screen, and the `appinstalled` event. Use when a site ships a `link rel="manifest"` and users must be able to install it - especially after a manifest, icon, or `start_url` edit that could silently drop install eligibility.