[@mantine/hooks] use-intersection: Accept a ref object for the root option

[h-harsh]

Closes #9026

useIntersection's root option previously only accepted a resolved Element, so the idiomatic React source — a ref — couldn't be used directly. ref.current is null on the first render and reading it during render is fragile, which is exactly what the documented usage example did (root: containerRef.current).

This mirrors #9031, which added ref support to useScrollSpy's scrollHost.

The catch (why this isn't a one-liner)

useScrollSpy attaches its listener inside a useEffect, so ref.current is already populated when it runs. useIntersection is different: it built the IntersectionObserver synchronously inside the target's callback ref. Because React attaches child refs before their parent's, a root ref is still null at that moment — so naively resolving root.current there would reproduce the exact bug, just relocated.

The fix moves observer creation into a useEffect (runs after all refs are committed), tracking the observed element in state via the callback ref. As a bonus this removes the old throwaway viewport-rooted observer + extra render that the previous root: ref.current pattern caused.

Changes

• Widen the option type via a new, exported UseIntersectionOptions (root?: IntersectionObserverInit['root'] | React.RefObject<Element | null>).
• Resolve a ref object to its element before constructing the observer; passing a resolved Element is unchanged.
• Add the hook's first test file — root resolution (element / ref / null-current / viewport default), option forwarding, entry updates, cleanup on detach/unmount, and an end-to-end render proving the ref root resolves correctly despite child-before-parent ref attachment order.
• Update the docs and usage demo to pass the ref directly (root: containerRef).

Notes

• Backwards compatible — existing Element / no-arg callers are unaffected.
• The usage-demo update overlaps with the docs-only PR docs(hooks): update useIntersection root example #9027; this supersedes it by demonstrating the new ref API directly.

Verification

• use-intersection tests: 11 passing
• tsc --noEmit clean, oxlint clean, formatted
• @mantine/hooks builds (incl. .d.ts generation)