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/is-arrayish/README.md

@@ -0,0 +1,16 @@
+# node-is-arrayish [![Travis-CI.org Build Status](https://img.shields.io/travis/Qix-/node-is-arrayish.svg?style=flat-square)](https://travis-ci.org/Qix-/node-is-arrayish) [![Coveralls.io Coverage Rating](https://img.shields.io/coveralls/Qix-/node-is-arrayish.svg?style=flat-square)](https://coveralls.io/r/Qix-/node-is-arrayish)
+> Determines if an object can be used like an Array
+
+## Example
+```javascript
+var isArrayish = require('is-arrayish');
+
+isArrayish([]); // true
+isArrayish({__proto__: []}); // true
+isArrayish({}); // false
+isArrayish({length:10}); // false
+```
+
+## License
+Licensed under the [MIT License](http://opensource.org/licenses/MIT).
+You can find a copy of it in [LICENSE](LICENSE).