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-normalize-string/README.md

@@ -0,0 +1,59 @@
+# [postcss][postcss]-normalize-string
+
+> Normalize strings with PostCSS.
+
+## Install
+
+With [npm](https://npmjs.org/package/postcss-normalize-string) do:
+
+```
+npm install postcss-normalize-string --save
+```
+
+## Example
+
+### Input
+
+```css
+p:after{ content: '\\'string\\' is intact' }
+```
+
+### Output
+
+```css
+p:after{ content:"'string' is intact" }
+```
+
+## Usage
+
+See the [PostCSS documentation](https://github.com/postcss/postcss#usage) for
+examples for your environment.
+
+## API
+
+### normalize([options])
+
+#### options
+
+##### preferredQuote
+
+Type: `string`
+Default: `double`
+
+Sets what type of quote to prefer. Possible values are `single` and `double`.
+
+```js
+var css = 'p:after{content:""}';
+console.log(postcss(normalize({preferredQuote: 'single'})).process(css).css);
+//=> p:after{content:''}
+```
+
+## 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