fix: use onMount return for cleanup instead of onDestroy

onDestroy runs during SSR in Svelte 5, causing document reference
errors. The onMount return function only runs on client teardown.

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

opal-web/src/lib/components/BottomSheet.svelte

@@ -1,5 +1,5 @@
 <script>
-	import { onMount, onDestroy, afterUpdate } from 'svelte';
+	import { onMount, afterUpdate } from 'svelte';
 
 	/** @type {boolean} */
 	export let open = false;
@@ -72,11 +72,11 @@
 	onMount(() => {
 		mounted = true;
 		document.addEventListener('keydown', handleKeydown);
-	});
 
-	onDestroy(() => {
+		return () => {
 			document.removeEventListener('keydown', handleKeydown);
 			document.documentElement.style.overflow = '';
+		};
 	});
 
 	/**