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.string.pad-end.js

@@ -0,0 +1,12 @@
+'use strict';
+var $ = require('../internals/export');
+var $padEnd = require('../internals/string-pad').end;
+var WEBKIT_BUG = require('../internals/string-pad-webkit-bug');
+
+// `String.prototype.padEnd` method
+// https://tc39.es/ecma262/#sec-string.prototype.padend
+$({ target: 'String', proto: true, forced: WEBKIT_BUG }, {
+  padEnd: function padEnd(maxLength /* , fillString = ' ' */) {
+    return $padEnd(this, maxLength, arguments.length > 1 ? arguments[1] : undefined);
+  }
+});