68Build infinite scrolling using IntersectionObserver

Build <InfiniteList fetchPage={fetchPage} />. fetchPage(page) (pages start at 1) resolves to { items: string[], hasMore: boolean }.

• Load page 1 on mount.
• Render the items as <li> elements, followed by an empty sentinel <div data-testid="sentinel" /> while there are more pages.
• When the sentinel scrolls into view (use IntersectionObserver with rootMargin: "200px"), load the next page and append its items.
• Show <p role="status">Loading…</p> while a page loads, and <p>You have seen everything</p> when hasMore is false.
• On failure, show <p role="alert"> with a Retry button that loads the same page again.

Requirements

• Never load the same page twice, even if the observer fires several times while a page is loading.
• Stop observing once there are no more pages.
• Disconnect the observer on unmount.