[FEAT] Provide a theme based on french design system

We should provide a basic theme based on https://www.systeme-de-design.gouv.fr/ .
Either by reimplementing the login theme or using https://github.com/dataesr/react-dsfr and https://github.com/InseeFrLab/keycloakify

Review the existing login theme and compare the French design system with the react-dsfr and keycloakify approaches linked in the issue. Decide which approach can provide a basic French design system-based theme, then verify that the resulting login theme is available and visually follows the referenced design system.

Written by the indexing model from the issue text.