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/stylelint/lib/utils/putIfAbsent.js

@@ -0,0 +1,22 @@
+'use strict';
+
+/**
+ * If `map` already has the given `key`, returns its value. Otherwise, calls
+ * `callback`, adds the result to `map` at `key`, and then returns it.
+ *
+ * @template K
+ * @template V
+ * @param {Map<K, V>} map
+ * @param {K} key
+ * @param {() => V} callback
+ * @returns {V}
+ */
+module.exports = function putIfAbsent(map, key, callback) {
+	if (map.has(key)) return /** @type {V} */ (map.get(key));
+
+	const value = callback();
+
+	map.set(key, value);
+
+	return value;
+};