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.string.link.js

@@ -0,0 +1,12 @@
+'use strict';
+var $ = require('../internals/export');
+var createHTML = require('../internals/create-html');
+var forcedStringHTMLMethod = require('../internals/string-html-forced');
+
+// `String.prototype.link` method
+// https://tc39.es/ecma262/#sec-string.prototype.link
+$({ target: 'String', proto: true, forced: forcedStringHTMLMethod('link') }, {
+  link: function link(url) {
+    return createHTML(this, 'a', 'href', url);
+  }
+});