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/conversations/+page.svelte

@@ -3,6 +3,7 @@
     import PersonTypeahead from '$lib/components/PersonTypeahead.svelte';
     import { untrack } from 'svelte';
     import { m } from '$lib/paraglide/messages.js';
+    import { formatDate } from '$lib/utils/date';
 
     let { data } = $props();
 
@@ -231,7 +232,7 @@
 											: 'text-gray-500'}"
 									>
 										<span class="flex items-center">
-											{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>
 										{#if doc.location}
 											<span class="flex items-center">