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/@hapi/hoek/lib/flatten.js

@@ -0,0 +1,20 @@
+'use strict';
+
+const internals = {};
+
+
+module.exports = internals.flatten = function (array, target) {
+
+    const result = target || [];
+
+    for (const entry of array) {
+        if (Array.isArray(entry)) {
+            internals.flatten(entry, result);
+        }
+        else {
+            result.push(entry);
+        }
+    }
+
+    return result;
+};