Extracted the grid mapping logic out of the return block into a `useMemo`
hook depending on `[assets, layout, openLightbox]`. This prevents the
entire photo grid—including complex Next.js `<Image>` and `<FadeIn>`
components—from needlessly re-rendering every time the user navigates
via the lightbox arrows or presses next/prev buttons, which previously
caused `lightboxIndex` to update and trigger a re-render of the entire grid.
Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
