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/icons/src/library/update.js

@@ -0,0 +1,12 @@
+/**
+ * WordPress dependencies
+ */
+import { SVG, Path } from '@wordpress/primitives';
+
+const update = (
+	<SVG xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24">
+		<Path d="m11.3 17.2-5-5c-.1-.1-.1-.3 0-.4l2.3-2.3-1.1-1-2.3 2.3c-.7.7-.7 1.8 0 2.5l5 5H7.5v1.5h5.3v-5.2h-1.5v2.6zm7.5-6.4-5-5h2.7V4.2h-5.2v5.2h1.5V6.8l5 5c.1.1.1.3 0 .4l-2.3 2.3 1.1 1.1 2.3-2.3c.6-.7.6-1.9-.1-2.5z" />
+	</SVG>
+);
+
+export default update;