Sanitize hostname generation from VM_NAME

Replace the previous simple space-removal with stricter sanitization when deriving the hostname from VM_NAME. Non-alphanumeric/hyphen sequences are collapsed to a single hyphen and leading/trailing hyphens are trimmed, preserving lowercase and ensuring a cleaner, more valid hostname string.

vm/archlinux-vm.sh

@@ -310,7 +310,10 @@ function advanced_settings() {
       HN="arch-linux"
       echo -e "${HOSTNAME}${BOLD}${DGN}Hostname: ${BGN}$HN${CL}"
     else
-      HN=$(echo "${VM_NAME,,}" | tr -d ' ')
+      HN=$(echo "${VM_NAME,,}" | tr -cs 'a-z0-9-' '-' | sed 's/^-//;s/-$//')
+      if [ "$HN" != "${VM_NAME,,}" ]; then
+        whiptail --backtitle "Proxmox VE Helper Scripts" --title "HOSTNAME ADJUSTED" --msgbox "Invalid characters detected. Hostname has been adjusted to:\n\n  $HN" 10 58
+      fi
       echo -e "${HOSTNAME}${BOLD}${DGN}Hostname: ${BGN}$HN${CL}"
     fi
   else