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-pure/internals/regexp-get-flags.js

@@ -0,0 +1,16 @@
+'use strict';
+var call = require('../internals/function-call');
+var hasOwn = require('../internals/has-own-property');
+var isPrototypeOf = require('../internals/object-is-prototype-of');
+var regExpFlagsDetection = require('../internals/regexp-flags-detection');
+var regExpFlagsGetterImplementation = require('../internals/regexp-flags');
+
+var RegExpPrototype = RegExp.prototype;
+
+module.exports = regExpFlagsDetection.correct ? function (it) {
+  return it.flags;
+} : function (it) {
+  return (!regExpFlagsDetection.correct && isPrototypeOf(RegExpPrototype, it) && !hasOwn(it, 'flags'))
+    ? call(regExpFlagsGetterImplementation, it)
+    : it.flags;
+};