Phase 4 cleanup & polish: param migration seeds, gnexus-ui-kit LoginPage styles, single .env ...

- Replace raw string interpolation in migration with $tb->prepare() + execute()

- Remove all var(--color-*) from LoginPage.vue in favor of gnexus-ui-kit utility classes

- Unify .env: single server/.env, remove server/SHServ/.env override, update config.php

- Update auth-fix-plan.md: mark Phase 4 tasks complete