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/esnext.bigint.range.js

@@ -0,0 +1,15 @@
+'use strict';
+/* eslint-disable es/no-bigint -- safe */
+var $ = require('../internals/export');
+var NumericRangeIterator = require('../internals/numeric-range-iterator');
+
+// `BigInt.range` method
+// https://github.com/tc39/proposal-iterator.range
+// TODO: Remove from `core-js@4`
+if (typeof BigInt == 'function') {
+  $({ target: 'BigInt', stat: true, forced: true }, {
+    range: function range(start, end, option) {
+      return new NumericRangeIterator(start, end, option, 'bigint', BigInt(0), BigInt(1));
+    }
+  });
+}