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.object.to-string.js

@@ -0,0 +1,10 @@
+'use strict';
+var TO_STRING_TAG_SUPPORT = require('../internals/to-string-tag-support');
+var defineBuiltIn = require('../internals/define-built-in');
+var toString = require('../internals/object-to-string');
+
+// `Object.prototype.toString` method
+// https://tc39.es/ecma262/#sec-object.prototype.tostring
+if (!TO_STRING_TAG_SUPPORT) {
+  defineBuiltIn(Object.prototype, 'toString', toString, { unsafe: true });
+}