fix: add beforeunload listener to game page

src/routes/game/+page.svelte

@@ -2,6 +2,7 @@
 	import type { CheckData, CheckResponse, ClientGameData, MapData } from "$lib/types";
 	import { goto } from "$app/navigation";
 	import { page } from "$app/stores";
+	import { onMount } from "svelte";
 	import L from "leaflet";
 
 	import "leaflet/dist/leaflet.css";
@@ -23,6 +24,10 @@
 	let solutionMarker: L.Marker | null = $state(null);
 	let solutionLine: L.Polyline | null = $state(null);
 
+	onMount(() => {
+		window.addEventListener("beforeunload", (e) => e.preventDefault());
+	});
+
 	async function fetchMap(): Promise<MapData> {
 		return fetch("/api/map?" + $page.url.searchParams).then((r) => r.json());
 	}