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/modules/es.object.lookup-getter.js

@@ -0,0 +1,23 @@
+'use strict';
+var $ = require('../internals/export');
+var DESCRIPTORS = require('../internals/descriptors');
+var FORCED = require('../internals/object-prototype-accessors-forced');
+var toObject = require('../internals/to-object');
+var toPropertyKey = require('../internals/to-property-key');
+var getPrototypeOf = require('../internals/object-get-prototype-of');
+var getOwnPropertyDescriptor = require('../internals/object-get-own-property-descriptor').f;
+
+// `Object.prototype.__lookupGetter__` method
+// https://tc39.es/ecma262/#sec-object.prototype.__lookupGetter__
+if (DESCRIPTORS) {
+  $({ target: 'Object', proto: true, forced: FORCED }, {
+    __lookupGetter__: function __lookupGetter__(P) {
+      var O = toObject(this);
+      var key = toPropertyKey(P);
+      var desc;
+      do {
+        if (desc = getOwnPropertyDescriptor(O, key)) return desc.get;
+      } while (O = getPrototypeOf(O));
+    }
+  });
+}