feat(documents): restore sender/receiver grouping in document list

When sort=SENDER, documents group under the sender's display name card.
When sort=RECEIVER, a document appears under each receiver's card
(with multi-receiver duplication). Falls back to i18n labels for unknown
sender/receiver. Passes sort prop from /documents page to DocumentList.

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

frontend/src/routes/documents/+page.svelte

@@ -119,5 +119,6 @@ $effect(() => {
 		q={data.q}
 		canWrite={data.canWrite}
 		error={data.error}
+		sort={sort}
 	/>
 </main>