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/array-buffer-is-detached.js

@@ -0,0 +1,17 @@
+'use strict';
+var globalThis = require('../internals/global-this');
+var NATIVE_ARRAY_BUFFER = require('../internals/array-buffer-basic-detection');
+var arrayBufferByteLength = require('../internals/array-buffer-byte-length');
+
+var DataView = globalThis.DataView;
+
+module.exports = function (O) {
+  if (!NATIVE_ARRAY_BUFFER || arrayBufferByteLength(O) !== 0) return false;
+  try {
+    // eslint-disable-next-line no-new -- thrower
+    new DataView(O);
+    return false;
+  } catch (error) {
+    return true;
+  }
+};