Fix 404 emoji: use duck emoji that renders on all systems

frontend/app/not-found.tsx

@@ -54,7 +54,7 @@ export default function NotFound() {
                     <Disc3 className="h-32 w-32 text-primary/20 group-hover:text-primary/40 transition-colors" />
                 </div>
                 <span className="absolute top-1/2 left-1/2 -translate-x-1/2 -translate-y-1/2 text-6xl group-hover:scale-110 transition-transform">
-                    🪿
+                    🦆
                 </span>
             </div>