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/@wordpress/browserslist-config/README.md

@@ -0,0 +1,39 @@
+# Browserslist Config
+
+[WordPress Browserslist](https://make.wordpress.org/core/handbook/best-practices/browser-support/) shareable config for [Browserslist](https://www.npmjs.com/package/browserslist).
+
+## Installation
+
+Install the module
+
+```shell
+$ npm install browserslist @wordpress/browserslist-config --save-dev
+```
+
+**Note**: This package requires Node.js 14.0.0 or later. It is not compatible with older versions.
+
+## Usage
+
+Add this to your `package.json` file:
+
+```json
+"browserslist": [
+	"extends @wordpress/browserslist-config"
+]
+```
+
+Alternatively, add this to `.browserslistrc` file:
+
+```
+extends @wordpress/browserslist-config
+```
+
+This package when imported returns an array of supported browsers, for more configuration examples including Autoprefixer, Babel, ESLint, PostCSS, and stylelint see the [Browserslist examples](https://github.com/ai/browserslist-example#browserslist-example) repo.
+
+## Contributing to this package
+
+This is an individual package that's part of the Gutenberg project. The project is organized as a monorepo. It's made up of multiple self-contained software packages, each with a specific purpose. The packages in this monorepo are published to [npm](https://www.npmjs.com/) and used by [WordPress](https://make.wordpress.org/core/) as well as other software projects.
+
+To find out more about contributing to this package or Gutenberg as a whole, please read the project's main [contributor guide](https://github.com/WordPress/gutenberg/tree/HEAD/CONTRIBUTING.md).
+
+<br /><br /><p align="center"><img src="https://s.w.org/style/images/codeispoetry.png?1" alt="Code is Poetry." /></p>