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/yearsToMonths/index.js

@@ -0,0 +1,24 @@
+import requiredArgs from "../_lib/requiredArgs/index.js";
+import { monthsInYear } from "../constants/index.js";
+/**
+ * @name yearsToMonths
+ * @category Conversion Helpers
+ * @summary Convert years to months.
+ *
+ * @description
+ * Convert a number of years to a full number of months.
+ *
+ * @param {number} years - number of years to be converted
+ *
+ * @returns {number} the number of years converted in months
+ * @throws {TypeError} 1 argument required
+ *
+ * @example
+ * // Convert 2 years into months
+ * const result = yearsToMonths(2)
+ * //=> 24
+ */
+export default function yearsToMonths(years) {
+  requiredArgs(1, arguments);
+  return Math.floor(years * monthsInYear);
+}