fix: fix partner checkbox default value (#17)

Przycisk był wyłączony pomimo uzupełnionych danych:
<img width="1404" height="656" alt="image"
src="https://github.com/user-attachments/assets/c23053b4-8874-4c00-9efa-1490efdf487c"
/>

Author: AdiPol1359

src/components/event-form/use-event-form.ts

@@ -10,7 +10,10 @@ import { eventFormSchema } from './event-form-schema';
 
 export const useEventForm = () => {
 	const thumbnailRef = useRef<HTMLDivElement>(null);
-	const form = useForm({ resolver: zodResolver(eventFormSchema) });
+	const form = useForm({
+		defaultValues: { withPartners: false },
+		resolver: zodResolver(eventFormSchema),
+	});
 
 	const isCompleted = form.formState.isValid;