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/esnext.map.update.js

@@ -0,0 +1,27 @@
+'use strict';
+var $ = require('../internals/export');
+var aCallable = require('../internals/a-callable');
+var aMap = require('../internals/a-map');
+var MapHelpers = require('../internals/map-helpers');
+
+var $TypeError = TypeError;
+var get = MapHelpers.get;
+var has = MapHelpers.has;
+var set = MapHelpers.set;
+
+// `Map.prototype.update` method
+// https://github.com/tc39/proposal-collection-methods
+$({ target: 'Map', proto: true, real: true, forced: true }, {
+  update: function update(key, callback /* , thunk */) {
+    var map = aMap(this);
+    var length = arguments.length;
+    aCallable(callback);
+    var isPresentInMap = has(map, key);
+    if (!isPresentInMap && length < 3) {
+      throw new $TypeError('Updating absent value');
+    }
+    var value = isPresentInMap ? get(map, key) : aCallable(length > 2 ? arguments[2] : undefined)(key, map);
+    set(map, key, callback(value, key, map));
+    return map;
+  }
+});