Move the Set-Cookie parser out of login/+page.server.ts into a shared module
with its own Vitest coverage (single-header, multi-header getSetCookie path,
missing-header, attribute-stripping, prefix-match-rejection). An Undici or
Node upgrade that changes header shape now trips its own test instead of
silently breaking login. Addresses PR #612 / Felix F2.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
