refactor(domain-manager): simplify frontend URL configuration (#10194)

Replaced multiple environment variables for frontend URL construction
with a single FRONTEND_URL variable. This change reduces complexity and
improves clarity by consolidating frontend URL handling into one source.
Updated relevant validations and removed unused variables like
FRONT_PROTOCOL and FRONT_PORT.

Fix #10016

packages/twenty-server/.env.example

@@ -5,9 +5,7 @@ REDIS_URL=redis://localhost:6379
 APP_SECRET=replace_me_with_a_random_string
 SIGN_IN_PREFILLED=true
 
-FRONT_PROTOCOL=http
-FRONT_DOMAIN=localhost
-FRONT_PORT=3001
+FRONTEND_URL=http://localhost:3001
 
 # ———————— Optional ————————
 # PORT=3000