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/url/src/get-authority.js

@@ -0,0 +1,21 @@
+/**
+ * Returns the authority part of the URL.
+ *
+ * @param {string} url The full URL.
+ *
+ * @example
+ * ```js
+ * const authority1 = getAuthority( 'https://wordpress.org/help/' ); // 'wordpress.org'
+ * const authority2 = getAuthority( 'https://localhost:8080/test/' ); // 'localhost:8080'
+ * ```
+ *
+ * @return {string|void} The authority part of the URL.
+ */
+export function getAuthority( url ) {
+	const matches = /^[^\/\s:]+:(?:\/\/)?\/?([^\/\s#?]+)[\/#?]{0,1}\S*$/.exec(
+		url
+	);
+	if ( matches ) {
+		return matches[ 1 ];
+	}
+}