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/scripts/utils/file.js

@@ -0,0 +1,39 @@
+/**
+ * External dependencies
+ */
+const { existsSync, readdirSync } = require( 'fs' );
+const path = require( 'path' );
+
+/**
+ * Internal dependencies
+ */
+const { getPackagePath } = require( './package' );
+
+const fromProjectRoot = ( fileName ) =>
+	path.join( path.dirname( getPackagePath() ), fileName );
+
+const hasProjectFile = ( fileName ) =>
+	existsSync( fromProjectRoot( fileName ) );
+
+const fromConfigRoot = ( fileName ) =>
+	path.join( path.dirname( __dirname ), 'config', fileName );
+
+const fromScriptsRoot = ( scriptName ) =>
+	path.join( path.dirname( __dirname ), 'scripts', `${ scriptName }.js` );
+
+const hasScriptFile = ( scriptName ) =>
+	existsSync( fromScriptsRoot( scriptName ) );
+
+const getScripts = () =>
+	readdirSync( path.join( path.dirname( __dirname ), 'scripts' ) )
+		.filter( ( f ) => path.extname( f ) === '.js' )
+		.map( ( f ) => path.basename( f, '.js' ) );
+
+module.exports = {
+	fromProjectRoot,
+	fromConfigRoot,
+	fromScriptsRoot,
+	getScripts,
+	hasProjectFile,
+	hasScriptFile,
+};