♻️(frontend) get theme from config endpoint

We centralized the configuration on the backend
side, it is easier to manage and we can change
the configuration without having to rebuild the
frontend.
We now use the config endpoint to get the theme,
we refacto to remove the frontend env occurences
and to adapt with the new way to get the theme.

src/frontend/Dockerfile

@@ -61,9 +61,6 @@ FROM impress AS impress-builder
 
 WORKDIR /home/frontend/apps/impress
 
-ARG FRONTEND_THEME
-ENV NEXT_PUBLIC_THEME=${FRONTEND_THEME}
-
 ARG Y_PROVIDER_URL
 ENV NEXT_PUBLIC_Y_PROVIDER_URL=${Y_PROVIDER_URL}