fix: format document date with Intl API (20. Dezember 2026)

Use Intl.DateTimeFormat with de-DE locale to show full German month
name. Appending T12:00:00 to the ISO string avoids UTC-midnight
timezone shifts misrendering the day.

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

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

@@ -151,7 +151,7 @@
 							</span>
 							<div>
 								<span class="block font-serif text-lg text-brand-navy">
-									{doc.documentDate ? doc.documentDate.split('-').reverse().join('.') : '—'}
+									{doc.documentDate ? new Intl.DateTimeFormat('de-DE', { day: 'numeric', month: 'long', year: 'numeric' }).format(new Date(doc.documentDate + 'T12:00:00')) : '—'}
 								</span>
 								<span class="text-xs font-sans text-gray-500">Dokumentendatum</span>
 							</div>