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.math.iaddh.js

@@ -0,0 +1,14 @@
+'use strict';
+var $ = require('../internals/export');
+
+// `Math.iaddh` method
+// https://gist.github.com/BrendanEich/4294d5c212a6d2254703
+// TODO: Remove from `core-js@4`
+$({ target: 'Math', stat: true, forced: true }, {
+  iaddh: function iaddh(x0, x1, y0, y1) {
+    var $x0 = x0 >>> 0;
+    var $x1 = x1 >>> 0;
+    var $y0 = y0 >>> 0;
+    return $x1 + (y1 >>> 0) + (($x0 & $y0 | ($x0 | $y0) & ~($x0 + $y0 >>> 0)) >>> 31) | 0;
+  }
+});