add ensure_whiptail function

This is required as some arm64 systems will not have whiptail installed, as it is not installed by default.

misc/build.func

@@ -2939,6 +2939,7 @@ install_script() {
   pve_check
   shell_check
   root_check
+  ensure_whiptail
   arch_check
   ssh_check
   maxkeys_check
@@ -5074,8 +5075,6 @@ create_lxc_container() {
     msg_ok "Downloaded ARM64 LXC template"
   }
 
-  # Architecture-aware template download wrapper.
-  # Optional $1 overrides destination path (for local-storage fallback).
   download_template() {
     local dest="${1:-$TEMPLATE_PATH}"
     if [[ "$ARCH" == "arm64" ]]; then

misc/core.func

@@ -1718,6 +1718,25 @@ function get_lxc_ip() {
   export LOCAL_IP
 }
 
+# ------------------------------------------------------------------------------
+# ensure_whiptail()
+#
+# - Ensures whiptail is installed
+# - Some ARM64 systems will not have whiptail installed.
+# - Exits with error message if installation fails
+# ------------------------------------------------------------------------------
+ensure_whiptail() {
+  command -v whiptail >/dev/null 2>&1 && return 0
+
+  msg_info "Installing whiptail"
+  apt_update_safe
+  $STD apt-get install -y whiptail || {
+    msg_error "Failed to install whiptail"
+    exit 115
+  }
+  msg_ok "Installed whiptail"
+}
+
 # ==============================================================================
 # SIGNAL TRAPS
 # ==============================================================================