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/utils/emitter.js

@@ -0,0 +1,46 @@
+/**
+ * Create an event emitter.
+ *
+ * @return {import("../types").DataEmitter} Emitter.
+ */
+export function createEmitter() {
+	let isPaused = false;
+	let isPending = false;
+	const listeners = new Set();
+	const notifyListeners = () =>
+		// We use Array.from to clone the listeners Set
+		// This ensures that we don't run a listener
+		// that was added as a response to another listener.
+		Array.from( listeners ).forEach( ( listener ) => listener() );
+
+	return {
+		get isPaused() {
+			return isPaused;
+		},
+
+		subscribe( listener ) {
+			listeners.add( listener );
+			return () => listeners.delete( listener );
+		},
+
+		pause() {
+			isPaused = true;
+		},
+
+		resume() {
+			isPaused = false;
+			if ( isPending ) {
+				isPending = false;
+				notifyListeners();
+			}
+		},
+
+		emit() {
+			if ( isPaused ) {
+				isPending = true;
+				return;
+			}
+			notifyListeners();
+		},
+	};
+}