feat(login): show xGat3 button only when accessed via IP address

Hide the Open xGat3 button when the page is accessed via a domain name.
Only show it when the hostname is a raw IP (e.g. 10.0.0.1 over WireGuard).

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

frontend/src/pages/Login/index.tsx

@@ -198,6 +198,7 @@ export default function Login() {
 					</div>
 				</div>
 				<div className="text-center text-secondary mt-3">{getVersion()}</div>
+				{/^\d+\.\d+\.\d+\.\d+$/.test(window.location.hostname) && (
 					<div className="text-center mt-3">
 						<a
 							href={`${window.location.origin}/wg-public`}
@@ -208,6 +209,7 @@ export default function Login() {
 							Open xGat3
 						</a>
 					</div>
+				)}
 			</div>
 		</Page>
 	);