fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/core-js/internals/object-property-is-enumerable.js

@@ -0,0 +1,14 @@
+'use strict';
+var $propertyIsEnumerable = {}.propertyIsEnumerable;
+// eslint-disable-next-line es/no-object-getownpropertydescriptor -- safe
+var getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
+
+// Nashorn ~ JDK8 bug
+var NASHORN_BUG = getOwnPropertyDescriptor && !$propertyIsEnumerable.call({ 1: 2 }, 1);
+
+// `Object.prototype.propertyIsEnumerable` method implementation
+// https://tc39.es/ecma262/#sec-object.prototype.propertyisenumerable
+exports.f = NASHORN_BUG ? function propertyIsEnumerable(V) {
+  var descriptor = getOwnPropertyDescriptor(this, V);
+  return !!descriptor && descriptor.enumerable;
+} : $propertyIsEnumerable;