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 };
 }