feat(login): add Open xGat3 button on login page

Adds a button below the login card that opens the xGat3 reverse
proxy interface (port 80) in a new tab when accessed via WireGuard VPN.
The link is built dynamically from the current hostname so it works
regardless of which IP the client uses to reach the server.

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

frontend/src/pages/Login/index.tsx

@@ -198,6 +198,16 @@ export default function Login() {
 					</div>
 				</div>
 				<div className="text-center text-secondary mt-3">{getVersion()}</div>
+				<div className="text-center mt-3">
+					<a
+						href={`http://${window.location.hostname}`}
+						className="btn btn-outline-secondary btn-sm"
+						target="_blank"
+						rel="noopener noreferrer"
+					>
+						Open xGat3
+					</a>
+				</div>
 			</div>
 		</Page>
 	);