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/use-lilius/node_modules/date-fns/secondsToMilliseconds.js

@@ -0,0 +1,24 @@
+"use strict";
+exports.secondsToMilliseconds = secondsToMilliseconds;
+var _index = require("./constants.js");
+
+/**
+ * @name secondsToMilliseconds
+ * @category Conversion Helpers
+ * @summary Convert seconds to milliseconds.
+ *
+ * @description
+ * Convert a number of seconds to a full number of milliseconds.
+ *
+ * @param seconds - The number of seconds to be converted
+ *
+ * @returns The number of seconds converted in milliseconds
+ *
+ * @example
+ * // Convert 2 seconds into milliseconds
+ * const result = secondsToMilliseconds(2)
+ * //=> 2000
+ */
+function secondsToMilliseconds(seconds) {
+  return seconds * _index.millisecondsInSecond;
+}