test+fix(docs): correct fallbackLabel when sort prop is omitted

Add failing test for DATE-sort + undated doc showing "Undatiert" fallback
label, then fix DocumentList by null-coalescing sort before comparison
((sort ?? 'DATE') === 'DATE'). Test uses one dated + one undated doc to
produce two groups and trigger GroupDivider rendering.

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

frontend/src/routes/DocumentList.svelte

@@ -30,7 +30,9 @@ let {
 	sort?: string;
 } = $props();
 
-const fallbackLabel = $derived(sort === 'DATE' ? m.docs_group_undated() : m.docs_group_unknown());
+const fallbackLabel = $derived(
+	(sort ?? 'DATE') === 'DATE' ? m.docs_group_undated() : m.docs_group_unknown()
+);
 const groupedDocuments = $derived.by(() =>
 	groupDocuments(documents, sort ?? 'DATE', fallbackLabel)
 );