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/globalthis/implementation.browser.js

@@ -0,0 +1,11 @@
+/* eslint no-negated-condition: 0, no-new-func: 0 */
+
+'use strict';
+
+if (typeof self !== 'undefined') {
+	module.exports = self;
+} else if (typeof window !== 'undefined') {
+	module.exports = window;
+} else {
+	module.exports = Function('return this')();
+}