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/create-property.js

@@ -0,0 +1,9 @@
+'use strict';
+var DESCRIPTORS = require('../internals/descriptors');
+var definePropertyModule = require('../internals/object-define-property');
+var createPropertyDescriptor = require('../internals/create-property-descriptor');
+
+module.exports = function (object, key, value) {
+  if (DESCRIPTORS) definePropertyModule.f(object, key, createPropertyDescriptor(0, value));
+  else object[key] = value;
+};