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/postcss-selector-parser/README.md

@@ -0,0 +1,49 @@
+# postcss-selector-parser [![test](https://github.com/postcss/postcss-selector-parser/actions/workflows/test.yml/badge.svg)](https://github.com/postcss/postcss-selector-parser/actions/workflows/test.yml)
+
+> Selector parser with built in methods for working with selector strings.
+
+## Install
+
+With [npm](https://npmjs.com/package/postcss-selector-parser) do:
+
+```
+npm install postcss-selector-parser
+```
+
+## Quick Start
+
+```js
+const parser = require('postcss-selector-parser');
+const transform = selectors => {
+    selectors.walk(selector => {
+        // do something with the selector
+        console.log(String(selector))
+    });
+};
+
+const transformed = parser(transform).processSync('h1, h2, h3');
+```
+
+To normalize selector whitespace:
+
+```js
+const parser = require('postcss-selector-parser');
+const normalized = parser().processSync('h1, h2, h3', {lossless: false});
+// -> h1,h2,h3
+```
+
+Async support is provided through `parser.process` and will resolve a Promise
+with the resulting selector string.
+
+## API
+
+Please see [API.md](API.md).
+
+## Credits
+
+* Huge thanks to Andrey Sitnik (@ai) for work on PostCSS which helped
+  accelerate this module's development.
+
+## License
+
+MIT