[@mantine/hooks] use-scroll-spy: Accept a ref object for scrollHost option

[Hossam-Ismail]

Closes #9025

Currently useScrollSpy's scrollHost option only accepts a resolved HTMLElement. The idiomatic way to obtain a DOM node in React is a ref, but a ref's .current is undefined on first render, so users have to convert the element into state via a callback ref just to make the hook re-initialize once the host exists:

const [scrollHost, setScrollHost] = useState<HTMLElement | null>(null);
useScrollSpy({ scrollHost: scrollHost ?? undefined, selector });
return <form ref={setScrollHost}>…</form>;

This change lets scrollHost accept either a resolved element or a ref object, collapsing the usage to the natural form:

const scrollHostRef = useRef<HTMLDivElement>(null);
useScrollSpy({ scrollHost: scrollHostRef, selector });
return <div ref={scrollHostRef}>…</div>;

Because the listener is attached inside useEffect (which runs after the DOM is committed), ref.current is already populated by the time the host is resolved, so no extra state/re-render is required. This mirrors the existing pattern of other hooks such as use-scroll-into-view, which already accept a RefObject.

Changes

• Widen scrollHost type to HTMLElement | RefObject<HTMLElement | null>.
• Resolve .current inside the effect when a ref object is passed; fall back to window when the resolved host is null/undefined (unchanged behavior for the existing HTMLElement and no-arg cases).
• Update the JSDoc for the option.

Tests

Added use-scroll-spy.test.ts covering: default window host, a resolved HTMLElement host, a ref-object host, and a ref whose current is null (falls back to window).