Theme polish: tasks

Work through the tasks in order; each ends with its checks passing and a commit. Tick a task when it is committed.

1. Spec: requirements, design and tasks.
2. Web app messages: Notyf at the top centre in the theme's colors.
3. Theme cookie: the web app saves the chosen theme for the login pages.
4. Login theme: the script, the three themes' styles, logo and board masks; checked in Chrome in all three themes, at desktop and phone width, with and without the cookie.
5. Docs: AGENTS.md (specs list, the cookie), apps/keycloak/README.md, TODO.md.