next@15.5.x hard-pins postcss 8.4.31, so a plain lockfile update cannot
reach the fix. Add a pnpm override (postcss@<8.5.18 -> 8.5.28, same major,
already resolved for vite) so the whole tree uses one patched postcss.
Lockfile diff is limited to dropping postcss 8.4.31 and repointing next.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>