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/es.array.last-index-of.js

@@ -0,0 +1,10 @@
+'use strict';
+var $ = require('../internals/export');
+var lastIndexOf = require('../internals/array-last-index-of');
+
+// `Array.prototype.lastIndexOf` method
+// https://tc39.es/ecma262/#sec-array.prototype.lastindexof
+// eslint-disable-next-line es/no-array-prototype-lastindexof -- required for testing
+$({ target: 'Array', proto: true, forced: lastIndexOf !== [].lastIndexOf }, {
+  lastIndexOf: lastIndexOf
+});