fix: destroy Packery instance on unmount to stop memory leak in SponsorBubbles

[VoxelFS]

from claude:

Summary

Fixes a memory leak on the home page caused by SponsorBubbles creating a new
Packery layout instance on every mount and on every baseSize change without
ever destroying the previous one. Each navigation back to / left behind a
full Packery instance plus all its DOM references, which Chrome's heap profiler
showed as a steadily growing pile of detached nodes and PackeryItem objects.

Root cause

In src/app/(frontend)/(home)/_components/SponsorBubbles.tsx, the Packery
effect was calling new Packery(...) and discarding the return value:

new Packery(containerRef.current, { ... });

Packery attaches listeners to the container and holds references to every
matched grid item. Without a handle to the instance, there was no way to call
.destroy(), so each remount stacked another live instance on top of the
previous one.

A secondary issue: the scroll-listener effect listed hoveredSponsor in its
dependency array, so the listener was being torn down and re-attached on every
hover change.

Changes

• Store the Packery instance in a useRef and call .destroy() in the effect
  cleanup (runs on unmount and before re-init when baseSize changes).
• Add a cancelled flag so a late-resolving dynamic import('packery') does
  not construct an orphaned instance after unmount.
• Switch the scroll handler to a functional setHoveredSponsor update and drop
  hoveredSponsor from the effect deps so the listener attaches once.

[koutaroyumiba]

lgtm - what a cooked bug