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/icon/index.js

@@ -0,0 +1,27 @@
+/**
+ * WordPress dependencies
+ */
+import { cloneElement, forwardRef } from '@wordpress/element';
+
+/** @typedef {{icon: JSX.Element, size?: number} & import('@wordpress/primitives').SVGProps} IconProps */
+
+/**
+ * Return an SVG icon.
+ *
+ * @param {IconProps}                                 props icon is the SVG component to render
+ *                                                          size is a number specifiying the icon size in pixels
+ *                                                          Other props will be passed to wrapped SVG component
+ * @param {import('react').ForwardedRef<HTMLElement>} ref   The forwarded ref to the SVG element.
+ *
+ * @return {JSX.Element}  Icon component
+ */
+function Icon( { icon, size = 24, ...props }, ref ) {
+	return cloneElement( icon, {
+		width: size,
+		height: size,
+		...props,
+		ref,
+	} );
+}
+
+export default forwardRef( Icon );