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/es-abstract/2023/IteratorStep.js

@@ -0,0 +1,21 @@
+'use strict';
+
+var $TypeError = require('es-errors/type');
+
+var IteratorComplete = require('./IteratorComplete');
+var IteratorNext = require('./IteratorNext');
+
+var isIteratorRecord = require('../helpers/records/iterator-record-2023');
+
+// https://262.ecma-international.org/14.0/#sec-iteratorstep
+
+module.exports = function IteratorStep(iteratorRecord) {
+	if (!isIteratorRecord(iteratorRecord)) {
+		throw new $TypeError('Assertion failed: `iteratorRecord` must be an Iterator Record'); // step 1
+	}
+
+	var result = IteratorNext(iteratorRecord); // step 1
+	var done = IteratorComplete(result); // step 2
+	return done === true ? false : result; // steps 3-4
+};
+