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/dom/src/test/utils/create-element.js

@@ -0,0 +1,55 @@
+/**
+ * Given an element type, returns an HTMLElement with an emulated layout,
+ * since JSDOM does have its own internal layout engine.
+ *
+ * @param {string} type Element type.
+ *
+ * @return {HTMLElement} Layout-emulated element.
+ */
+export default function createElement( type ) {
+	const element = document.createElement( type );
+
+	const ifNotHidden = ( value, elseValue ) =>
+		function () {
+			let isHidden = false;
+			let node = this;
+			do {
+				isHidden =
+					node.style.display === 'none' ||
+					node.style.visibility === 'hidden';
+
+				node = node.parentNode;
+			} while (
+				! isHidden &&
+				node &&
+				node.nodeType === node.ELEMENT_NODE
+			);
+
+			return isHidden ? elseValue : value;
+		};
+
+	Object.defineProperties( element, {
+		offsetHeight: {
+			get: ifNotHidden( 10, 0 ),
+		},
+		offsetWidth: {
+			get: ifNotHidden( 10, 0 ),
+		},
+	} );
+
+	element.getClientRects = ifNotHidden(
+		[
+			{
+				width: 10,
+				height: 10,
+				top: 0,
+				right: 10,
+				bottom: 10,
+				left: 0,
+			},
+		],
+		[]
+	);
+
+	return element;
+}