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/persons/+page.server.ts

@@ -1,12 +1,18 @@
+import { error } from '@sveltejs/kit';
 import { createApiClient } from '$lib/api.server';
+import { getErrorMessage } from '$lib/errors';
 
 export async function load({ url, fetch }) {
     const q = url.searchParams.get('q') || '';
     const api = createApiClient(fetch);
 
-    const { data } = await api.GET('/api/persons', {
+    const result = await api.GET('/api/persons', {
         params: { query: { q: q || undefined } }
     });
 
-    return { persons: data ?? [], q };
+    if (!result.response.ok) {
+        throw error(result.response.status, getErrorMessage(undefined));
+    }
+
+    return { persons: result.data!, q };
 }

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

@@ -3,6 +3,7 @@
     import PersonTypeahead from '$lib/components/PersonTypeahead.svelte';
     import { m } from '$lib/paraglide/messages.js';
     import { sortDocumentsByDate, type SortDir } from '$lib/utils/sort';
+    import { formatDate } from '$lib/utils/date';
 
     let { data, form } = $props();
 
@@ -38,7 +39,7 @@
     const sentYearRange = $derived(yearRange(sentDocuments));
     const receivedYearRange = $derived(yearRange(receivedDocuments));
 
-    const coCorrespondents = $derived(() => {
+    const coCorrespondents = $derived.by(() => {
         const freq = new Map<string, { id: string; name: string; count: number }>();
 
         for (const doc of sentDocuments) {
@@ -312,11 +313,11 @@
     {/if}
 
     <!-- Co-Correspondents Section -->
-    {#if coCorrespondents().length > 0}
+    {#if coCorrespondents.length > 0}
     <div class="mb-6">
         <h3 class="text-xs font-bold uppercase tracking-widest text-gray-400 mb-3">{m.person_co_correspondents_heading()}</h3>
         <div class="flex flex-wrap gap-2">
-            {#each coCorrespondents() as c}
+            {#each coCorrespondents as c}
                 <a href="/conversations?senderId={person.id}&receiverId={c.id}"
                    class="inline-flex items-center gap-1.5 px-3 py-1 rounded-full border border-brand-sand text-sm font-serif text-brand-navy hover:border-brand-navy transition-colors">
                     {c.name}
@@ -366,7 +367,7 @@
                                             {doc.title || doc.originalFilename}
                                         </div>
                                         <div class="flex items-center text-xs font-sans text-gray-500 mt-0.5 space-x-2">
-                                            <span>{doc.documentDate ? new Intl.DateTimeFormat('de-DE', { day: 'numeric', month: 'long', year: 'numeric' }).format(new Date(doc.documentDate + 'T12:00:00')) : m.doc_no_date()}</span>
+                                            <span>{doc.documentDate ? formatDate(doc.documentDate) : m.doc_no_date()}</span>
                                             {#if doc.location}
                                                 <span class="text-brand-mint">•</span>
                                                 <span>{doc.location}</span>
@@ -438,7 +439,7 @@
                                             {doc.title || doc.originalFilename}
                                         </div>
                                         <div class="flex items-center text-xs font-sans text-gray-500 mt-0.5 space-x-2">
-                                            <span>{doc.documentDate ? new Intl.DateTimeFormat('de-DE', { day: 'numeric', month: 'long', year: 'numeric' }).format(new Date(doc.documentDate + 'T12:00:00')) : m.doc_no_date()}</span>
+                                            <span>{doc.documentDate ? formatDate(doc.documentDate) : m.doc_no_date()}</span>
                                             {#if doc.location}
                                                 <span class="text-brand-mint">•</span>
                                                 <span>{doc.location}</span>