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/modules/esnext.array.last-index.js

@@ -0,0 +1,22 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var DESCRIPTORS = require('../internals/descriptors');
+var addToUnscopables = require('../internals/add-to-unscopables');
+var toObject = require('../internals/to-object');
+var lengthOfArrayLike = require('../internals/length-of-array-like');
+var defineBuiltInAccessor = require('../internals/define-built-in-accessor');
+
+// `Array.prototype.lastIndex` getter
+// https://github.com/tc39/proposal-array-last
+if (DESCRIPTORS) {
+  defineBuiltInAccessor(Array.prototype, 'lastIndex', {
+    configurable: true,
+    get: function lastIndex() {
+      var O = toObject(this);
+      var len = lengthOfArrayLike(O);
+      return len === 0 ? 0 : len - 1;
+    }
+  });
+
+  addToUnscopables('lastIndex');
+}