refactor(admin): move SMTP settings from user settings to admin area

SMTP configuration is an admin concern, not a per-user setting. Removed
it from the settings sidebar and relocated to /admin/smtp with a link in
the admin sidebar.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/messages/de.json

@@ -74,7 +74,8 @@
       "users": "Benutzer",
       "tenants": "Mandanten",
       "modules": "Module",
-      "ldap": "LDAP"
+      "ldap": "LDAP",
+      "smtp": "SMTP"
     },
     "role": {
       "SUPER_ADMIN": "Super-Admin",