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/axios/lib/helpers/isAxiosError.js

@@ -0,0 +1,14 @@
+'use strict';
+
+import utils from '../utils.js';
+
+/**
+ * Determines whether the payload is an error thrown by Axios
+ *
+ * @param {*} payload The value to test
+ *
+ * @returns {boolean} True if the payload is an error thrown by Axios, otherwise false
+ */
+export default function isAxiosError(payload) {
+  return utils.isObject(payload) && payload.isAxiosError === true;
+}