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/reakit/es/Provider.js

@@ -0,0 +1,18 @@
+import { createElement } from 'react';
+import { unstable_IdProvider } from './Id/IdProvider.js';
+import { SystemProvider } from 'reakit-system/SystemProvider';
+
+var defaultSystem = {};
+function Provider(_ref) {
+  var prefix = _ref.unstable_prefix,
+      _ref$unstable_system = _ref.unstable_system,
+      system = _ref$unstable_system === void 0 ? defaultSystem : _ref$unstable_system,
+      children = _ref.children;
+  return /*#__PURE__*/createElement(unstable_IdProvider, {
+    prefix: prefix
+  }, /*#__PURE__*/createElement(SystemProvider, {
+    unstable_system: system
+  }, children));
+}
+
+export { Provider };