combobox: improve props, fix placement with scroll

src/lib/Checkbox.svelte

@@ -5,6 +5,7 @@
 <script lang="ts">
 	import type { Snippet } from 'svelte';
 	import { validate } from '@svelte-toolkit/validate';
+	import { generateIdentifier } from './util.js';
 
 	interface Props {
 		name?: string;
@@ -24,9 +25,7 @@
 		onchange
 	}: Props = $props();
 
-	let id = $derived.by(() => {
-		return `checkbox-${name || Math.random().toString(36).substring(2, 15)}`;
-	});
+	let id = $derived(generateIdentifier('checkbox', name));
 
 	let valid = $state(true);
 </script>