perf(web): preload landing hero background (MUL-6711) (#7582)

Preload the above-the-fold landing background so it starts downloading alongside
the product preview instead of after it, shortening the black-fallback window on
cold loads. Also declares an explicit `sizes="100vw"` on the backdrop and moves
the product preview off the deprecated `priority` prop onto `preload`.

Co-authored-by: Binteng <89759610+Bbbtt04@users.noreply.github.com>