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/object-define-properties.js

@@ -0,0 +1,21 @@
+'use strict';
+var DESCRIPTORS = require('../internals/descriptors');
+var V8_PROTOTYPE_DEFINE_BUG = require('../internals/v8-prototype-define-bug');
+var definePropertyModule = require('../internals/object-define-property');
+var anObject = require('../internals/an-object');
+var toIndexedObject = require('../internals/to-indexed-object');
+var objectKeys = require('../internals/object-keys');
+
+// `Object.defineProperties` method
+// https://tc39.es/ecma262/#sec-object.defineproperties
+// eslint-disable-next-line es/no-object-defineproperties -- safe
+exports.f = DESCRIPTORS && !V8_PROTOTYPE_DEFINE_BUG ? Object.defineProperties : function defineProperties(O, Properties) {
+  anObject(O);
+  var props = toIndexedObject(Properties);
+  var keys = objectKeys(Properties);
+  var length = keys.length;
+  var index = 0;
+  var key;
+  while (length > index) definePropertyModule.f(O, key = keys[index++], props[key]);
+  return O;
+};