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/dom-ready/src/index.js

@@ -0,0 +1,45 @@
+/**
+ * @typedef {() => void} Callback
+ *
+ * TODO: Remove this typedef and inline `() => void` type.
+ *
+ * This typedef is used so that a descriptive type is provided in our
+ * automatically generated documentation.
+ *
+ * An in-line type `() => void` would be preferable, but the generated
+ * documentation is `null` in that case.
+ *
+ * @see https://github.com/WordPress/gutenberg/issues/18045
+ */
+
+/**
+ * Specify a function to execute when the DOM is fully loaded.
+ *
+ * @param {Callback} callback A function to execute after the DOM is ready.
+ *
+ * @example
+ * ```js
+ * import domReady from '@wordpress/dom-ready';
+ *
+ * domReady( function() {
+ * 	//do something after DOM loads.
+ * } );
+ * ```
+ *
+ * @return {void}
+ */
+export default function domReady( callback ) {
+	if ( typeof document === 'undefined' ) {
+		return;
+	}
+
+	if (
+		document.readyState === 'complete' || // DOMContentLoaded + Images/Styles/etc loaded, so we call directly.
+		document.readyState === 'interactive' // DOMContentLoaded fires at this point, so we call directly.
+	) {
+		return void callback();
+	}
+
+	// DOMContentLoaded has not fired yet, delay callback until then.
+	document.addEventListener( 'DOMContentLoaded', callback );
+}

node_modules/@wordpress/dom-ready/src/test/index.test.js

@@ -0,0 +1,49 @@
+/**
+ * Internal dependencies
+ */
+import domReady from '../';
+
+describe( 'domReady', () => {
+	beforeAll( () => {
+		Object.defineProperty( document, 'readyState', {
+			value: 'loading',
+			writable: true,
+		} );
+	} );
+
+	describe( 'when document readystate is complete', () => {
+		it( 'should call the callback.', () => {
+			const callback = jest.fn( () => {} );
+			document.readyState = 'complete';
+			domReady( callback );
+			expect( callback ).toHaveBeenCalled();
+		} );
+	} );
+
+	describe( 'when document readystate is interactive', () => {
+		it( 'should call the callback.', () => {
+			const callback = jest.fn( () => {} );
+			document.readyState = 'interactive';
+			domReady( callback );
+			expect( callback ).toHaveBeenCalled();
+		} );
+	} );
+
+	describe( 'when document readystate is still loading', () => {
+		it( 'should add the callback as an event listener to the DOMContentLoaded event.', () => {
+			const addEventListener = jest.fn( () => {} );
+			document.readyState = 'loading';
+			Object.defineProperty( document, 'addEventListener', {
+				value: addEventListener,
+			} );
+
+			const callback = jest.fn( () => {} );
+			domReady( callback );
+			expect( callback ).not.toHaveBeenCalled();
+			expect( addEventListener ).toHaveBeenCalledWith(
+				'DOMContentLoaded',
+				callback
+			);
+		} );
+	} );
+} );