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/istanbul-reports/index.js

@@ -0,0 +1,24 @@
+'use strict';
+/*
+ Copyright 2012-2015, Yahoo Inc.
+ Copyrights licensed under the New BSD License. See the accompanying LICENSE file for terms.
+ */
+const path = require('path');
+
+module.exports = {
+    create(name, cfg) {
+        cfg = cfg || {};
+        let Cons;
+        try {
+            Cons = require(path.join(__dirname, 'lib', name));
+        } catch (e) {
+            if (e.code !== 'MODULE_NOT_FOUND') {
+                throw e;
+            }
+
+            Cons = require(name);
+        }
+
+        return new Cons(cfg);
+    }
+};