refactor(frontend): apply formatDate utility and fix derived/error handling

- Replace 5 inline Intl.DateTimeFormat blocks with formatDate() across
  home, conversations, persons detail, and document detail pages
- Fix coCorrespondents: $derived(() => ...) → $derived.by(...) —
  the old form typed the value as a function, breaking template call sites
- Persons list: throw error on API failure instead of silently returning []

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

frontend/src/routes/documents/[id]/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
     import { m } from '$lib/paraglide/messages.js';
+    import { formatDate } from '$lib/utils/date';
 
     let { data } = $props();
 
@@ -119,7 +120,7 @@
 							</span>
 							<div>
 								<span class="block font-serif text-lg text-brand-navy">
-									{doc.documentDate ? new Intl.DateTimeFormat('de-DE', { day: 'numeric', month: 'long', year: 'numeric' }).format(new Date(doc.documentDate + 'T12:00:00')) : '—'}
+									{doc.documentDate ? formatDate(doc.documentDate) : '—'}
 								</span>
 								<span class="text-xs font-sans text-gray-500">{m.doc_label_document_date()}</span>
 							</div>