fix(react): Restore isIntersecting and unsubscribed on unmount (#1325)

[M-Blobby]

Summary

• Fix VideoTrack visibility tracking by wiring useIntersectionObserver correctly with intersectionRef
• Restore reliable intersectionEntry?.isIntersecting updates for manageSubscription
• Handle GridLayout pagination/unmount case by unsubscribing on tile unmount, since intersection "exit" is not guaranteed there

I hope my analysis is correct and that this contribution will also be helpful to others.

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