revert: DNS_SERVER default back to udp://1.1.1.1

10.35.99.172 is a local-environment resolver, not suitable as a
public default. On networks where public UDP DNS is unreachable,
pass DNS_SERVER explicitly at runtime.

config_builder.py

@@ -56,7 +56,7 @@ def build_singbox_config(
     socks_port: int = 1080,
     socks_user: str = "",
     socks_pass: str = "",
-    dns_server: str = "udp://10.35.99.172",
+    dns_server: str = "udp://1.1.1.1",
     log_level: str = "info",
 ) -> Dict[str, Any]:
     """Build a complete sing-box JSON configuration.