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/+page.svelte

@@ -5,6 +5,7 @@ import TagInput from '$lib/components/TagInput.svelte';
 import { slide } from 'svelte/transition';
 import { untrack } from 'svelte';
 import { m } from '$lib/paraglide/messages.js';
+import { formatDate } from '$lib/utils/date';
 
 let { data } = $props();
 
@@ -240,7 +241,7 @@ $effect(() => {
 									<div class="mb-4 flex flex-wrap gap-6 font-sans text-sm text-gray-500">
 										<div class="flex items-center">
 											<img src="/degruyter-icons/Simple/Medium-24px/SVG/Action/Calendar/Calendar-Add-MD.svg" alt="" aria-hidden="true" class="mr-1.5 h-4 w-4" />
-											{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) : '—'}
 										</div>
 										{#if doc.location}
 											<div class="flex items-center">