🔧(backend) add FRONTEND_THEME setting

The frontend need to know the theme to be used,
so we need to add a new setting to the backend,
in order to expose this value to the frontend.

env.d/development/common.dist

@@ -48,3 +48,6 @@ AI_MODEL=llama
 
 # Collaboration
 COLLABORATION_SERVER_URL=ws://localhost:4444
+
+# Frontend
+FRONTEND_THEME=dsfr