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/to-positive-integer.js

@@ -0,0 +1,10 @@
+'use strict';
+var toIntegerOrInfinity = require('../internals/to-integer-or-infinity');
+
+var $RangeError = RangeError;
+
+module.exports = function (it) {
+  var result = toIntegerOrInfinity(it);
+  if (result < 0) throw new $RangeError("The argument can't be less than 0");
+  return result;
+};