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/2015/IsConcatSpreadable.js

@@ -0,0 +1,26 @@
+'use strict';
+
+var GetIntrinsic = require('get-intrinsic');
+
+var $isConcatSpreadable = GetIntrinsic('%Symbol.isConcatSpreadable%', true);
+
+var Get = require('./Get');
+var IsArray = require('./IsArray');
+var ToBoolean = require('./ToBoolean');
+
+var isObject = require('es-object-atoms/isObject');
+
+// https://262.ecma-international.org/6.0/#sec-isconcatspreadable
+
+module.exports = function IsConcatSpreadable(O) {
+	if (!isObject(O)) {
+		return false;
+	}
+	if ($isConcatSpreadable) {
+		var spreadable = Get(O, $isConcatSpreadable);
+		if (typeof spreadable !== 'undefined') {
+			return ToBoolean(spreadable);
+		}
+	}
+	return IsArray(O);
+};