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.regexp.flags.js

@@ -0,0 +1,16 @@
+'use strict';
+var DESCRIPTORS = require('../internals/descriptors');
+var defineBuiltInAccessor = require('../internals/define-built-in-accessor');
+var regExpFlagsDetection = require('../internals/regexp-flags-detection');
+var regExpFlagsGetterImplementation = require('../internals/regexp-flags');
+
+// `RegExp.prototype.flags` getter
+// https://tc39.es/ecma262/#sec-get-regexp.prototype.flags
+if (DESCRIPTORS && !regExpFlagsDetection.correct) {
+  defineBuiltInAccessor(RegExp.prototype, 'flags', {
+    configurable: true,
+    get: regExpFlagsGetterImplementation
+  });
+
+  regExpFlagsDetection.correct = true;
+}