Register the beforeunload listener only while a beforeNavigate callback exists

_start_router registers beforeunload unconditionally (packages/kit/src/runtime/client/client.js:3164 at 2eed63867). Firefox excludes any page with a beforeunload listener from the bfcache, so every SvelteKit app loses back/forward cache there even when no beforeNavigate callback exists to run.

The listener exists to fire beforeNavigate with type: 'leave' and to reset history.scrollRestoration; persist_state already also runs on visibilitychange, which MDN recommends for state saving. Add the listener when the first beforeNavigate callback registers, remove it when the last one unregisters, and move the scrollRestoration reset to pagehide.

https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event#usage_notes