fix(live-preview): fallback serverURL to localhost instead of empty string

postMessage with empty string origin throws "Invalid target origin ''"
Use same fallback as payload.config.ts — http://localhost:3000.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/components/cms/RefreshRouteOnSave.tsx

@@ -8,7 +8,7 @@ export function RefreshRouteOnSave() {
   return (
     <PayloadLivePreview
       refresh={() => router.refresh()}
-      serverURL={process.env.NEXT_PUBLIC_SITE_URL ?? ''}
+      serverURL={process.env.NEXT_PUBLIC_SITE_URL ?? 'http://localhost:3000'}
     />
   )
 }