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/@wordpress/data/src/promise-middleware.js

@@ -0,0 +1,23 @@
+/**
+ * External dependencies
+ */
+import isPromise from 'is-promise';
+
+/**
+ * Simplest possible promise redux middleware.
+ *
+ * @type {import('redux').Middleware}
+ */
+const promiseMiddleware = () => ( next ) => ( action ) => {
+	if ( isPromise( action ) ) {
+		return action.then( ( resolvedAction ) => {
+			if ( resolvedAction ) {
+				return next( resolvedAction );
+			}
+		} );
+	}
+
+	return next( action );
+};
+
+export default promiseMiddleware;