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/a11y/src/clear.js

@@ -0,0 +1,16 @@
+/**
+ * Clears the a11y-speak-region elements and hides the explanatory text.
+ */
+export default function clear() {
+	const regions = document.getElementsByClassName( 'a11y-speak-region' );
+	const introText = document.getElementById( 'a11y-speak-intro-text' );
+
+	for ( let i = 0; i < regions.length; i++ ) {
+		regions[ i ].textContent = '';
+	}
+
+	// Make sure the explanatory text is hidden from assistive technologies.
+	if ( introText ) {
+		introText.setAttribute( 'hidden', 'hidden' );
+	}
+}