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.date.now.js

@@ -0,0 +1,15 @@
+'use strict';
+// TODO: Remove from `core-js@4`
+var $ = require('../internals/export');
+var uncurryThis = require('../internals/function-uncurry-this');
+
+var $Date = Date;
+var thisTimeValue = uncurryThis($Date.prototype.getTime);
+
+// `Date.now` method
+// https://tc39.es/ecma262/#sec-date.now
+$({ target: 'Date', stat: true }, {
+  now: function now() {
+    return thisTimeValue(new $Date());
+  }
+});