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/methods/index.js

@@ -0,0 +1,69 @@
+/*!
+ * methods
+ * Copyright(c) 2013-2014 TJ Holowaychuk
+ * Copyright(c) 2015-2016 Douglas Christopher Wilson
+ * MIT Licensed
+ */
+
+'use strict';
+
+/**
+ * Module dependencies.
+ * @private
+ */
+
+var http = require('http');
+
+/**
+ * Module exports.
+ * @public
+ */
+
+module.exports = getCurrentNodeMethods() || getBasicNodeMethods();
+
+/**
+ * Get the current Node.js methods.
+ * @private
+ */
+
+function getCurrentNodeMethods() {
+  return http.METHODS && http.METHODS.map(function lowerCaseMethod(method) {
+    return method.toLowerCase();
+  });
+}
+
+/**
+ * Get the "basic" Node.js methods, a snapshot from Node.js 0.10.
+ * @private
+ */
+
+function getBasicNodeMethods() {
+  return [
+    'get',
+    'post',
+    'put',
+    'head',
+    'delete',
+    'options',
+    'trace',
+    'copy',
+    'lock',
+    'mkcol',
+    'move',
+    'purge',
+    'propfind',
+    'proppatch',
+    'unlock',
+    'report',
+    'mkactivity',
+    'checkout',
+    'merge',
+    'm-search',
+    'notify',
+    'subscribe',
+    'unsubscribe',
+    'patch',
+    'search',
+    'connect'
+  ];
+}