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/warning/src/index.js

@@ -0,0 +1,50 @@
+/**
+ * Internal dependencies
+ */
+import { logged } from './utils';
+
+function isDev() {
+	return typeof SCRIPT_DEBUG !== 'undefined' && SCRIPT_DEBUG === true;
+}
+
+/**
+ * Shows a warning with `message` if environment is not `production`.
+ *
+ * @param {string} message Message to show in the warning.
+ *
+ * @example
+ * ```js
+ * import warning from '@wordpress/warning';
+ *
+ * function MyComponent( props ) {
+ *   if ( ! props.title ) {
+ *     warning( '`props.title` was not passed' );
+ *   }
+ *   ...
+ * }
+ * ```
+ */
+export default function warning( message ) {
+	if ( ! isDev() ) {
+		return;
+	}
+
+	// Skip if already logged.
+	if ( logged.has( message ) ) {
+		return;
+	}
+
+	// eslint-disable-next-line no-console
+	console.warn( message );
+
+	// Throwing an error and catching it immediately to improve debugging
+	// A consumer can use 'pause on caught exceptions'
+	// https://github.com/facebook/react/issues/4216
+	try {
+		throw Error( message );
+	} catch ( x ) {
+		// Do nothing.
+	}
+
+	logged.add( message );
+}

node_modules/@wordpress/warning/src/test/index.js

@@ -0,0 +1,36 @@
+/**
+ * Internal dependencies
+ */
+import warning from '..';
+import { logged } from '../utils';
+
+describe( 'warning', () => {
+	const initialScriptDebug = global.SCRIPT_DEBUG;
+
+	afterEach( () => {
+		global.SCRIPT_DEBUG = initialScriptDebug;
+		logged.clear();
+	} );
+
+	it( 'logs to console.warn when SCRIPT_DEBUG is set to `true`', () => {
+		global.SCRIPT_DEBUG = true;
+		warning( 'warning' );
+		expect( console ).toHaveWarnedWith( 'warning' );
+	} );
+
+	it( 'does not log to console.warn if SCRIPT_DEBUG not set to `true`', () => {
+		global.SCRIPT_DEBUG = false;
+		warning( 'warning' );
+		expect( console ).not.toHaveWarned();
+	} );
+
+	it( 'should show a message once', () => {
+		global.SCRIPT_DEBUG = true;
+		warning( 'warning' );
+		warning( 'warning' );
+
+		expect( console ).toHaveWarned();
+		// eslint-disable-next-line no-console
+		expect( console.warn ).toHaveBeenCalledTimes( 1 );
+	} );
+} );

node_modules/@wordpress/warning/src/utils.js

@@ -0,0 +1,7 @@
+/**
+ * Object map tracking messages which have been logged, for use in ensuring a
+ * message is only logged once.
+ *
+ * @type {Set<string>}
+ */
+export const logged = new Set();