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/object-keys-internal.js

@@ -0,0 +1,21 @@
+'use strict';
+var uncurryThis = require('../internals/function-uncurry-this');
+var hasOwn = require('../internals/has-own-property');
+var toIndexedObject = require('../internals/to-indexed-object');
+var indexOf = require('../internals/array-includes').indexOf;
+var hiddenKeys = require('../internals/hidden-keys');
+
+var push = uncurryThis([].push);
+
+module.exports = function (object, names) {
+  var O = toIndexedObject(object);
+  var i = 0;
+  var result = [];
+  var key;
+  for (key in O) !hasOwn(hiddenKeys, key) && hasOwn(O, key) && push(result, key);
+  // Don't enum bug & hidden keys
+  while (names.length > i) if (hasOwn(O, key = names[i++])) {
+    ~indexOf(result, key) || push(result, key);
+  }
+  return result;
+};