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/react-dom/client.js

@@ -0,0 +1,25 @@
+'use strict';
+
+var m = require('react-dom');
+if (process.env.NODE_ENV === 'production') {
+  exports.createRoot = m.createRoot;
+  exports.hydrateRoot = m.hydrateRoot;
+} else {
+  var i = m.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;
+  exports.createRoot = function(c, o) {
+    i.usingClientEntryPoint = true;
+    try {
+      return m.createRoot(c, o);
+    } finally {
+      i.usingClientEntryPoint = false;
+    }
+  };
+  exports.hydrateRoot = function(c, h, o) {
+    i.usingClientEntryPoint = true;
+    try {
+      return m.hydrateRoot(c, h, o);
+    } finally {
+      i.usingClientEntryPoint = false;
+    }
+  };
+}