fix: add missing is_domain helper function to x-ui.sh (#3612)

The is_domain function was being called in ssl_cert_issue() but was never
defined in x-ui.sh, causing 'Invalid domain format' errors for valid domains.

Added is_ipv4, is_ipv6, is_ip, and is_domain helper functions to match
the definitions in install.sh and update.sh.

Co-authored-by: wyatt <wyatt@Wyatts-MacBook-Air.local>

x-ui.sh

@@ -19,6 +19,20 @@ function LOGI() {
     echo -e "${green}[INF] $* ${plain}"
 }
 
+# Simple helpers for domain/IP validation
+is_ipv4() {
+    [[ "$1" =~ ^([0-9]{1,3}\.){3}[0-9]{1,3}$ ]] && return 0 || return 1
+}
+is_ipv6() {
+    [[ "$1" =~ : ]] && return 0 || return 1
+}
+is_ip() {
+    is_ipv4 "$1" || is_ipv6 "$1"
+}
+is_domain() {
+    [[ "$1" =~ ^([A-Za-z0-9](-*[A-Za-z0-9])*\.)+[A-Za-z]{2,}$ ]] && return 0 || return 1
+}
+
 # check root
 [[ $EUID -ne 0 ]] && LOGE "ERROR: You must be root to run this script! \n" && exit 1