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/es-abstract/2024/ClearKeptObjects.js

@@ -0,0 +1,12 @@
+'use strict';
+
+var SLOT = require('internal-slot');
+var keptObjects = [];
+
+// https://262.ecma-international.org/12.0/#sec-clear-kept-objects
+
+module.exports = function ClearKeptObjects() {
+	keptObjects.length = 0;
+};
+
+SLOT.set(module.exports, '[[es-abstract internal: KeptAlive]]', keptObjects);