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-merge-longhand/README.md

@@ -0,0 +1,50 @@
+# [postcss][postcss]-merge-longhand
+
+> Merge longhand properties into shorthand with PostCSS.
+
+## Install
+
+With [npm](https://npmjs.org/package/postcss-merge-longhand) do:
+
+```
+npm install postcss-merge-longhand --save
+```
+
+## Example
+
+Merge longhand properties into shorthand; works with `margin`, `padding` &
+`border`. For more examples see the [tests](src/__tests__/index.js).
+
+### Input
+
+```css
+h1 {
+    margin-top: 10px;
+    margin-right: 20px;
+    margin-bottom: 10px;
+    margin-left: 20px;
+}
+```
+
+### Output
+
+```css
+h1 {
+    margin: 10px 20px;
+}
+```
+
+## Usage
+
+See the [PostCSS documentation](https://github.com/postcss/postcss#usage) for
+examples for your environment.
+
+## Contributors
+
+See [CONTRIBUTORS.md](https://github.com/cssnano/cssnano/blob/master/CONTRIBUTORS.md).
+
+## License
+
+MIT © [Ben Briggs](http://beneb.info)
+
+[postcss]: https://github.com/postcss/postcss