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/@ungap/structured-clone/esm/index.js

@@ -0,0 +1,25 @@
+import {deserialize} from './deserialize.js';
+import {serialize} from './serialize.js';
+
+/**
+ * @typedef {Array<string,any>} Record a type representation
+ */
+
+/**
+ * Returns an array of serialized Records.
+ * @param {any} any a serializable value.
+ * @param {{transfer?: any[], json?: boolean, lossy?: boolean}?} options an object with
+ * a transfer option (ignored when polyfilled) and/or non standard fields that
+ * fallback to the polyfill if present.
+ * @returns {Record[]}
+ */
+export default typeof structuredClone === "function" ?
+  /* c8 ignore start */
+  (any, options) => (
+    options && ('json' in options || 'lossy' in options) ?
+      deserialize(serialize(any, options)) : structuredClone(any)
+  ) :
+  (any, options) => deserialize(serialize(any, options));
+  /* c8 ignore stop */
+
+export {deserialize, serialize};