fix: add isMounted for View component

[pqoqubbw]

Summary

Test Plan

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Note

Low Risk
Small, localized rendering-condition change intended to avoid SSR/hydration mismatches; main risk is unintentionally hiding a view until after first client render.

Overview
Updates View to gate rendering behind a client-side mounted flag set via useIsomorphicLayoutEffect, instead of relying on a memoized isVisible calculation.

This prevents the component from rendering during SSR/initial hydration when visibility can’t be reliably determined, returning null until both mounted and the matching item is active.

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