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/asynckit/serial.js

@@ -0,0 +1,17 @@
+var serialOrdered = require('./serialOrdered.js');
+
+// Public API
+module.exports = serial;
+
+/**
+ * Runs iterator over provided array elements in series
+ *
+ * @param   {array|object} list - array or object (named list) to iterate over
+ * @param   {function} iterator - iterator to run
+ * @param   {function} callback - invoked when all elements processed
+ * @returns {function} - jobs terminator
+ */
+function serial(list, iterator, callback)
+{
+  return serialOrdered(list, iterator, null, callback);
+}