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/gradient-parser/index.html

@@ -0,0 +1,12 @@
+<html>
+  <head>
+    <script src="build/web.js"></script>
+    <script>
+      document.addEventListener('DOMContentLoaded', function() {
+        document.body.innerHTML = '<pre>'+ JSON.stringify(GradientParser.parse('radial-gradient(ellipse cover, rgb(0, 57, 115), rgb(229, 229, 190))'), null, 2) +'</pre>';
+      });
+    </script>
+  </head>
+  <body>
+  </body>
+</html>