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/indexed-object.js

@@ -0,0 +1,16 @@
+'use strict';
+var uncurryThis = require('../internals/function-uncurry-this');
+var fails = require('../internals/fails');
+var classof = require('../internals/classof-raw');
+
+var $Object = Object;
+var split = uncurryThis(''.split);
+
+// fallback for non-array-like ES3 and non-enumerable old V8 strings
+module.exports = fails(function () {
+  // throws an error in rhino, see https://github.com/mozilla/rhino/issues/346
+  // eslint-disable-next-line no-prototype-builtins -- safe
+  return !$Object('z').propertyIsEnumerable(0);
+}) ? function (it) {
+  return classof(it) === 'String' ? split(it, '') : $Object(it);
+} : $Object;