feat(ui): style admin shell for retro theme and add prod docker config

- Update the retro theme CSS to style the dashboard, account, and admin pages with a classic Windows 98 aesthetic (silver sidebar, bevelled tabs, sunken metric cards).
- Exclude sidebar links and tabs from default retro link styles to ensure readability.
- Add `docker-compose-prod.yml` for production deployments.
- Add `.prod.env` to `.gitignore`.

.gitignore

@@ -12,5 +12,6 @@ backend/static/uploads/*
 .env
 .env.*
 !.env.example
+.prod.env
 scripts/env/dev.env
 docker-compose.yml