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/date-fns/esm/_lib/assign/index.js

@@ -0,0 +1,12 @@
+export default function assign(target, object) {
+  if (target == null) {
+    throw new TypeError('assign requires that input parameter not be null or undefined');
+  }
+  for (var property in object) {
+    if (Object.prototype.hasOwnProperty.call(object, property)) {
+      ;
+      target[property] = object[property];
+    }
+  }
+  return target;
+}