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/symbol-tree/lib/TreePosition.js

@@ -0,0 +1,11 @@
+'use strict';
+
+/* eslint-disable sort-keys */
+module.exports = Object.freeze({
+        // same as DOM DOCUMENT_POSITION_
+        DISCONNECTED: 1,
+        PRECEDING: 2,
+        FOLLOWING: 4,
+        CONTAINS: 8,
+        CONTAINED_BY: 16,
+});