refactor: simplify TrustHosts middleware and use APP_URL as base_url fallback

- Delegate host validation to parent class instead of custom implementation
- Update base_url() helper to use config('app.url') instead of url('/')
- Add test for APP_URL fallback when no FQDN or public IPs configured
- Remove dedicated TrustHostsMiddlewareTest (logic now tested via integration tests)

bootstrap/helpers/shared.php

@@ -491,7 +491,7 @@ function base_url(bool $withPort = true): string
         return "http://[$settings->public_ipv6]";
     }
 
-    return url('/');
+    return config('app.url');
 }
 
 function isSubscribed()