refactor(documents): narrow sort prop type to full SortMode union

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

frontend/src/routes/DocumentList.svelte

@@ -7,6 +7,8 @@ import type { components } from '$lib/generated/api';
 
 type DocumentSearchItem = components['schemas']['DocumentSearchItem'];
 
+type SortMode = 'DATE' | 'TITLE' | 'SENDER' | 'RECEIVER' | 'UPLOAD_DATE' | 'RELEVANCE';
+
 let {
 	items,
 	canWrite,
@@ -20,7 +22,7 @@ let {
 	error?: string | null;
 	total?: number;
 	q?: string;
-	sort?: string;
+	sort?: SortMode;
 } = $props();
 
 const groups = $derived.by(() => {