perf: replace scroll listener with IntersectionObserver

[airajena]

Concisely describe the change

The PlayOnScroll component was checking getBoundingClientRect() on every scroll event to determine whether the video should play or pause. Since the component is mounted four times on /features, this meant four synchronous layout reads were happening on every scroll.

Replaced the scroll listener with an IntersectionObserver using a threshold of 1, keeping the same fully-visible behavior while avoiding the repeated layout reads during scrolling.

Verified with yarn typecheck and yarn build, and manually checked /features to make sure videos still play and pause correctly while scrolling.

Fixes: #656

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