fix(i18n): add ERR_PROXY_PRIVATE_HOST translation + rename net-safety error codes

Dashboard i18n now includes ERR_PROXY_PRIVATE_HOST ("Private/local host is not allowed" / "内网/本地主机不允许"). net-safety.js error codes renamed from ERR_PRIVATE_HOST/ERR_PRIVATE_IP to ERR_PROXY_PRIVATE_HOST/ERR_PROXY_PRIVATE_IP for consistency with error.* namespace convention.