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/components/src/box-control/linked-button.tsx

@@ -0,0 +1,31 @@
+/**
+ * WordPress dependencies
+ */
+import { link, linkOff } from '@wordpress/icons';
+import { __ } from '@wordpress/i18n';
+
+/**
+ * Internal dependencies
+ */
+import Button from '../button';
+import Tooltip from '../tooltip';
+
+export default function LinkedButton( {
+	isLinked,
+	...props
+}: { isLinked?: boolean } & React.ComponentProps< typeof Button > ) {
+	const label = isLinked ? __( 'Unlink sides' ) : __( 'Link sides' );
+
+	return (
+		<Tooltip text={ label }>
+			<Button
+				{ ...props }
+				className="component-box-control__linked-button"
+				size="small"
+				icon={ isLinked ? link : linkOff }
+				iconSize={ 24 }
+				aria-label={ label }
+			/>
+		</Tooltip>
+	);
+}