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/lodash/stubArray.js

@@ -0,0 +1,23 @@
+/**
+ * This method returns a new empty array.
+ *
+ * @static
+ * @memberOf _
+ * @since 4.13.0
+ * @category Util
+ * @returns {Array} Returns the new empty array.
+ * @example
+ *
+ * var arrays = _.times(2, _.stubArray);
+ *
+ * console.log(arrays);
+ * // => [[], []]
+ *
+ * console.log(arrays[0] === arrays[1]);
+ * // => false
+ */
+function stubArray() {
+  return [];
+}
+
+module.exports = stubArray;