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/date-fns/esm/getUnixTime/index.js

@@ -0,0 +1,23 @@
+import getTime from "../getTime/index.js";
+import requiredArgs from "../_lib/requiredArgs/index.js";
+/**
+ * @name getUnixTime
+ * @category Timestamp Helpers
+ * @summary Get the seconds timestamp of the given date.
+ *
+ * @description
+ * Get the seconds timestamp of the given date.
+ *
+ * @param {Date|Number} date - the given date
+ * @returns {Number} the timestamp
+ * @throws {TypeError} 1 argument required
+ *
+ * @example
+ * // Get the timestamp of 29 February 2012 11:45:05 CET:
+ * const result = getUnixTime(new Date(2012, 1, 29, 11, 45, 5))
+ * //=> 1330512305
+ */
+export default function getUnixTime(dirtyDate) {
+  requiredArgs(1, arguments);
+  return Math.floor(getTime(dirtyDate) / 1000);
+}