ConstRegisters event listeners for beforeinstallprompt and appinstalled.
Shows a banner when the browser signals the PWA is installable.
Does nothing if the user has previously dismissed the prompt, and is idempotent
(a second call does not stack a second set of global listeners).
Returns true if the browser has a deferred install prompt ready.
Registry destroy / test seam: detaches the global listeners, hides the banner and clears the deferred prompt so a re-init starts clean (S7.5 — no listener leak).
Android PWA install prompt controller.