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-pure/modules/es.array.includes.js

@@ -0,0 +1,28 @@
+'use strict';
+var $ = require('../internals/export');
+var $includes = require('../internals/array-includes').includes;
+var fails = require('../internals/fails');
+var addToUnscopables = require('../internals/add-to-unscopables');
+
+// FF99+ bug
+var BROKEN_ON_SPARSE = fails(function () {
+  // eslint-disable-next-line es/no-array-prototype-includes -- detection
+  return !Array(1).includes();
+});
+
+// Safari 26.4- bug
+var BROKEN_ON_SPARSE_WITH_FROM_INDEX = fails(function () {
+  // eslint-disable-next-line no-sparse-arrays, es/no-array-prototype-includes -- detection
+  return [, 1].includes(undefined, 1);
+});
+
+// `Array.prototype.includes` method
+// https://tc39.es/ecma262/#sec-array.prototype.includes
+$({ target: 'Array', proto: true, forced: BROKEN_ON_SPARSE || BROKEN_ON_SPARSE_WITH_FROM_INDEX }, {
+  includes: function includes(el /* , fromIndex = 0 */) {
+    return $includes(this, el, arguments.length > 1 ? arguments[1] : undefined);
+  }
+});
+
+// https://tc39.es/ecma262/#sec-array.prototype-@@unscopables
+addToUnscopables('includes');