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-pure/internals/array-set-length.js

@@ -0,0 +1,27 @@
+'use strict';
+var DESCRIPTORS = require('../internals/descriptors');
+var isArray = require('../internals/is-array');
+
+var $TypeError = TypeError;
+// eslint-disable-next-line es/no-object-getownpropertydescriptor -- safe
+var getOwnPropertyDescriptor = Object.getOwnPropertyDescriptor;
+
+// Safari < 13 does not throw an error in this case
+var SILENT_ON_NON_WRITABLE_LENGTH_SET = DESCRIPTORS && !function () {
+  // makes no sense without proper strict mode support
+  if (this !== undefined) return true;
+  try {
+    // eslint-disable-next-line es/no-object-defineproperty -- safe
+    Object.defineProperty([], 'length', { writable: false }).length = 1;
+  } catch (error) {
+    return error instanceof TypeError;
+  }
+}();
+
+module.exports = SILENT_ON_NON_WRITABLE_LENGTH_SET ? function (O, length) {
+  if (isArray(O) && !getOwnPropertyDescriptor(O, 'length').writable) {
+    throw new $TypeError('Cannot set read only .length');
+  } return O.length = length;
+} : function (O, length) {
+  return O.length = length;
+};