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/html-entities/src/index.js

@@ -0,0 +1,61 @@
+/** @type {HTMLTextAreaElement} */
+let _decodeTextArea;
+
+/**
+ * Decodes the HTML entities from a given string.
+ *
+ * @param {string} html String that contain HTML entities.
+ *
+ * @example
+ * ```js
+ * import { decodeEntities } from '@wordpress/html-entities';
+ *
+ * const result = decodeEntities( 'á' );
+ * console.log( result ); // result will be "á"
+ * ```
+ *
+ * @return {string} The decoded string.
+ */
+export function decodeEntities( html ) {
+	// Not a string, or no entities to decode.
+	if ( 'string' !== typeof html || -1 === html.indexOf( '&' ) ) {
+		return html;
+	}
+
+	// Create a textarea for decoding entities, that we can reuse.
+	if ( undefined === _decodeTextArea ) {
+		if (
+			document.implementation &&
+			document.implementation.createHTMLDocument
+		) {
+			_decodeTextArea = document.implementation
+				.createHTMLDocument( '' )
+				.createElement( 'textarea' );
+		} else {
+			_decodeTextArea = document.createElement( 'textarea' );
+		}
+	}
+
+	_decodeTextArea.innerHTML = html;
+	const decoded = _decodeTextArea.textContent;
+	_decodeTextArea.innerHTML = '';
+
+	/**
+	 * Cast to string, HTMLTextAreaElement should always have `string` textContent.
+	 *
+	 * > The `textContent` property of the `Node` interface represents the text content of the
+	 * > node and its descendants.
+	 * >
+	 * > Value: A string or `null`
+	 * >
+	 * > * If the node is a `document` or a Doctype, `textContent` returns `null`.
+	 * > * If the node is a CDATA section, comment, processing instruction, or text node,
+	 * >   textContent returns the text inside the node, i.e., the `Node.nodeValue`.
+	 * > * For other node types, `textContent returns the concatenation of the textContent of
+	 * >   every child node, excluding comments and processing instructions. (This is an empty
+	 * >   string if the node has no children.)
+	 *
+	 * @see https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent
+	 */
+	return /** @type {string} */ ( decoded );
+}

node_modules/@wordpress/html-entities/src/test/entities.js

@@ -0,0 +1,27 @@
+/**
+ * Internal dependencies
+ */
+import { decodeEntities } from '../';
+
+describe( 'decodeEntities', () => {
+	it( 'should not change html with no entities', () => {
+		const html = '<h1>A noble tag embiggens the smallest text.</h1>';
+		const expected = '<h1>A noble tag embiggens the smallest text.</h1>';
+		expect( decodeEntities( html ) ).toEqual( expected );
+	} );
+	it( 'should decode entities', () => {
+		const html = '&lt;h1&gt;This post&#8217;s title.&lt;/h1&gt;';
+		const expected = '<h1>This post’s title.</h1>';
+		expect( decodeEntities( html ) ).toEqual( expected );
+	} );
+	it( 'should not double decode entities', () => {
+		const html = 'This post&amp;rsquo;s title.';
+		const expected = 'This post&rsquo;s title.';
+		expect( decodeEntities( html ) ).toEqual( expected );
+	} );
+	it( 'should not care about leading zeros on entity codes', () => {
+		const html = 'Jim&#0039;s mother&#039s post&#39s title.';
+		const expected = "Jim's mother's post's title.";
+		expect( decodeEntities( html ) ).toEqual( expected );
+	} );
+} );