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/eslint-plugin-react/lib/rules/no-find-dom-node.js

@@ -0,0 +1,56 @@
+/**
+ * @fileoverview Prevent usage of findDOMNode
+ * @author Yannick Croissant
+ */
+
+'use strict';
+
+const docsUrl = require('../util/docsUrl');
+const report = require('../util/report');
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+const messages = {
+  noFindDOMNode: 'Do not use findDOMNode. It doesn’t work with function components and is deprecated in StrictMode. See https://reactjs.org/docs/react-dom.html#finddomnode',
+};
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Disallow usage of findDOMNode',
+      category: 'Best Practices',
+      recommended: true,
+      url: docsUrl('no-find-dom-node'),
+    },
+
+    messages,
+
+    schema: [],
+  },
+
+  create(context) {
+    return {
+      CallExpression(node) {
+        const callee = node.callee;
+
+        const isFindDOMNode = ('name' in callee && callee.name === 'findDOMNode') || (
+          'property' in callee
+          && callee.property
+          && 'name' in callee.property
+          && callee.property.name === 'findDOMNode'
+        );
+
+        if (!isFindDOMNode) {
+          return;
+        }
+
+        report(context, messages.noFindDOMNode, 'noFindDOMNode', {
+          node: callee,
+        });
+      },
+    };
+  },
+};