ABN-298/feat: reset field values on "Use System Value" re-check

Snapshots field values when inherit checkbox is unchecked (field still shows
system value at that moment), restores on re-check. Triggers change event
so dependent visibility updates cascade.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dgjlindsay

view/adminhtml/web/js/payment-terms-config.js

@@ -131,11 +131,46 @@ define(['jquery', 'domReady!'], function ($) {
         $defaultTerm.on('change', onDefaultTermChanged);
         $('#two_payment_payment_terms_surcharge_type_inherit').on('change', onSurchargeChanged);
 
+        // ── "Use System Value" reset ────────────────────────────────────
+
+        function initInheritResetBehavior() {
+            var prefix = 'two_payment_payment_terms_';
+            $('input[id^="' + prefix + '"][id$="_inherit"]').each(function () {
+                var $inherit = $(this);
+                var fieldId = $inherit.attr('id').replace(/_inherit$/, '');
+                var $field = $('#' + fieldId);
+                if (!$field.length) {
+                    return;
+                }
+
+                var systemValue = null;
+
+                // If inherit is checked on load, current value IS the system value
+                if ($inherit.is(':checked')) {
+                    systemValue = $field.val();
+                }
+
+                $inherit.on('change', function () {
+                    if ($inherit.is(':checked')) {
+                        // Re-checking: restore system value if we have it
+                        if (systemValue !== null) {
+                            $field.val(systemValue);
+                            $field.trigger('change');
+                        }
+                    } else {
+                        // Unchecking: field still shows system value — snapshot it
+                        systemValue = $field.val();
+                    }
+                });
+            });
+        }
+
         // ── Initialize ───────────────────────────────────────────────────
 
         updateDefaultTermOptions();
         updateDifferentialOptionLabel();
         updateSurchargeVisibility();
+        initInheritResetBehavior();
     }
 
     $(document).ready(function () {