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/axios/lib/helpers/callbackify.js

@@ -0,0 +1,18 @@
+import utils from '../utils.js';
+
+const callbackify = (fn, reducer) => {
+  return utils.isAsyncFn(fn)
+    ? function (...args) {
+        const cb = args.pop();
+        fn.apply(this, args).then((value) => {
+          try {
+            reducer ? cb(null, ...reducer(value)) : cb(null, value);
+          } catch (err) {
+            cb(err);
+          }
+        }, cb);
+      }
+    : fn;
+};
+
+export default callbackify;