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/webpack-dev-server/client/utils/sendMessage.js

@@ -0,0 +1,16 @@
+/* global __resourceQuery WorkerGlobalScope */
+
+// Send messages to the outside, so plugins can consume it.
+/**
+ * @param {string} type
+ * @param {any} [data]
+ */
+function sendMsg(type, data) {
+  if (typeof self !== "undefined" && (typeof WorkerGlobalScope === "undefined" || !(self instanceof WorkerGlobalScope))) {
+    self.postMessage({
+      type: "webpack".concat(type),
+      data: data
+    }, "*");
+  }
+}
+export default sendMsg;