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/internals/iterate-simple.js

@@ -0,0 +1,12 @@
+'use strict';
+var call = require('../internals/function-call');
+
+module.exports = function (record, fn, ITERATOR_INSTEAD_OF_RECORD) {
+  var iterator = ITERATOR_INSTEAD_OF_RECORD ? record : record.iterator;
+  var next = record.next;
+  var step, result;
+  while (!(step = call(next, iterator)).done) {
+    result = fn(step.value);
+    if (result !== undefined) return result;
+  }
+};