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/es-abstract/2020/IsDataDescriptor.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var hasOwn = require('hasown');
+
+var isPropertyDescriptor = require('../helpers/records/property-descriptor');
+
+// https://262.ecma-international.org/5.1/#sec-8.10.2
+
+module.exports = function IsDataDescriptor(Desc) {
+	if (typeof Desc === 'undefined') {
+		return false;
+	}
+
+	if (!isPropertyDescriptor(Desc)) {
+		throw new $TypeError('Assertion failed: `Desc` must be a Property Descriptor');
+	}
+
+	if (!hasOwn(Desc, '[[Value]]') && !hasOwn(Desc, '[[Writable]]')) {
+		return false;
+	}
+
+	return true;
+};