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/esnext.observable.of.js

@@ -0,0 +1,24 @@
+'use strict';
+var $ = require('../internals/export');
+var getBuiltIn = require('../internals/get-built-in');
+var isConstructor = require('../internals/is-constructor');
+
+var Array = getBuiltIn('Array');
+
+// `Observable.of` method
+// https://github.com/tc39/proposal-observable
+$({ target: 'Observable', stat: true, forced: true }, {
+  of: function of() {
+    var C = isConstructor(this) ? this : getBuiltIn('Observable');
+    var length = arguments.length;
+    var items = Array(length);
+    var index = 0;
+    while (index < length) items[index] = arguments[index++];
+    return new C(function (observer) {
+      for (var i = 0; i < length; i++) {
+        observer.next(items[i]);
+        if (observer.closed) return;
+      } observer.complete();
+    });
+  }
+});