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/import-local/readme.md

@@ -0,0 +1,25 @@
+# import-local
+
+> Let a globally installed package use a locally installed version of itself if available
+
+Useful for CLI tools that want to defer to the user's locally installed version when available, but still work if it's not installed locally. For example, [AVA](https://avajs.dev) and [XO](https://github.com/xojs/xo) uses this method.
+
+## Install
+
+```sh
+npm install import-local
+```
+
+## Usage
+
+```js
+import importLocal from 'import-local';
+
+if (importLocal(import.meta.url)) {
+	console.log('Using local version of this package');
+} else {
+	// Code for both global and local version here…
+}
+```
+
+You can also pass in `__filename` when used in a CommonJS context.