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/modules/es.object.define-property.js

@@ -0,0 +1,11 @@
+'use strict';
+var $ = require('../internals/export');
+var DESCRIPTORS = require('../internals/descriptors');
+var defineProperty = require('../internals/object-define-property').f;
+
+// `Object.defineProperty` method
+// https://tc39.es/ecma262/#sec-object.defineproperty
+// eslint-disable-next-line es/no-object-defineproperty -- safe
+$({ target: 'Object', stat: true, forced: Object.defineProperty !== defineProperty, sham: !DESCRIPTORS }, {
+  defineProperty: defineProperty
+});