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/es-to-primitive/helpers/isPrimitive.js

@@ -0,0 +1,6 @@
+'use strict';
+
+/** @type {(value: unknown) => value is null | undefined | string | symbol | number | boolean | bigint} */
+module.exports = function isPrimitive(value) {
+	return value === null || (typeof value !== 'function' && typeof value !== 'object');
+};