Development preview only. Use this to polish the install experience before publishing.
Shows after the browser raises the native install prompt.
Pressing the button should open the browser's add-to-home-screen prompt.
Safari does not show the native prompt.
The button should display instructions: Share → Add to Home Screen.
The install button stays hidden once the app is already on the home screen.
This is the banner users will see when a new version is waiting in the service worker.