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-absolute-index.js

@@ -0,0 +1,13 @@
+'use strict';
+var toIntegerOrInfinity = require('../internals/to-integer-or-infinity');
+
+var max = Math.max;
+var min = Math.min;
+
+// Helper for a popular repeating case of the spec:
+// Let integer be ? ToInteger(index).
+// If integer < 0, let result be max((length + integer), 0); else let result be min(integer, length).
+module.exports = function (index, length) {
+  var integer = toIntegerOrInfinity(index);
+  return integer < 0 ? max(integer + length, 0) : min(integer, length);
+};