Refactor advanced_settings and add whitelist check

Moved and redefined _is_whitelisted_key as a global function for use in multiple locations. Improved formatting and readability in the advanced_settings wizard, including better handling of container type selection and IPv6 configuration steps.

misc/build.func

@@ -780,7 +780,13 @@ if ! declare -p VAR_WHITELIST >/dev/null 2>&1; then
   )
 fi
 
-# Note: _is_whitelisted_key() is defined above in default_var_settings section
+# Global whitelist check function (used by _load_vars_file_to_map and others)
+_is_whitelisted_key() {
+  local k="$1"
+  local w
+  for w in "${VAR_WHITELIST[@]}"; do [ "$k" = "$w" ] && return 0; done
+  return 1
+}
 
 _sanitize_value() {
   # Disallow Command-Substitution / Shell-Meta
@@ -1199,7 +1205,10 @@ advanced_settings() {
     1)
       local default_on="ON"
       local default_off="OFF"
-      [[ "$_ct_type" == "0" ]] && { default_on="OFF"; default_off="ON"; }
+      [[ "$_ct_type" == "0" ]] && {
+        default_on="OFF"
+        default_off="ON"
+      }
 
       if result=$(whiptail --backtitle "Proxmox VE Helper Scripts [Step $STEP/$MAX_STEP]" \
         --title "CONTAINER TYPE" \