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/jsx-uses-react.js

@@ -0,0 +1,52 @@
+/**
+ * @fileoverview Prevent React to be marked as unused
+ * @author Glen Mailer
+ */
+
+'use strict';
+
+const pragmaUtil = require('../util/pragma');
+const docsUrl = require('../util/docsUrl');
+const markVariableAsUsed = require('../util/eslint').markVariableAsUsed;
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+  // eslint-disable-next-line eslint-plugin/prefer-message-ids -- https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/issues/292
+  meta: {
+    docs: {
+      description: 'Disallow React to be incorrectly marked as unused',
+      category: 'Best Practices',
+      recommended: true,
+      url: docsUrl('jsx-uses-react'),
+    },
+    schema: [],
+  },
+
+  create(context) {
+    const pragma = pragmaUtil.getFromContext(context);
+    const fragment = pragmaUtil.getFragmentFromContext(context);
+
+    /**
+     * @param {ASTNode} node
+     * @returns {void}
+     */
+    function handleOpeningElement(node) {
+      markVariableAsUsed(pragma, node, context);
+    }
+    // --------------------------------------------------------------------------
+    // Public
+    // --------------------------------------------------------------------------
+
+    return {
+      JSXOpeningElement: handleOpeningElement,
+      JSXOpeningFragment: handleOpeningElement,
+      JSXFragment(node) {
+        markVariableAsUsed(fragment, node, context);
+      },
+    };
+  },
+};