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.get-year.js

@@ -0,0 +1,20 @@
+'use strict';
+var $ = require('../internals/export');
+var uncurryThis = require('../internals/function-uncurry-this');
+var fails = require('../internals/fails');
+
+// IE8- non-standard case
+var FORCED = fails(function () {
+  // eslint-disable-next-line es/no-date-prototype-getyear-setyear -- detection
+  return new Date(16e11).getYear() !== 120;
+});
+
+var getFullYear = uncurryThis(Date.prototype.getFullYear);
+
+// `Date.prototype.getYear` method
+// https://tc39.es/ecma262/#sec-date.prototype.getyear
+$({ target: 'Date', proto: true, forced: FORCED }, {
+  getYear: function getYear() {
+    return getFullYear(this) - 1900;
+  }
+});