feat(#145): internationalise dashboard widget strings (de/en/es)

Replace all hardcoded German strings in dashboard components with
Paraglide translation keys. Date locale uses getLocale() instead
of the hardcoded 'de-DE'.

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

frontend/src/lib/components/DashboardRecentDocuments.svelte

@@ -1,4 +1,7 @@
 <script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+import { getLocale } from '$lib/paraglide/runtime.js';
+
 type Document = {
 	id: string;
 	title: string;
@@ -13,7 +16,7 @@ interface Props {
 let { recentDocs }: Props = $props();
 
 function formatDate(dateStr: string): string {
-	return new Intl.DateTimeFormat('de-DE', {
+	return new Intl.DateTimeFormat(getLocale(), {
 		day: 'numeric',
 		month: 'long',
 		year: 'numeric'
@@ -24,7 +27,7 @@ function formatDate(dateStr: string): string {
 {#if recentDocs.length > 0}
 	<div data-testid="dashboard-recent-docs" class="rounded-sm border border-line bg-surface p-6">
 		<h2 class="mb-4 font-sans text-xs font-bold tracking-widest text-gray-400 uppercase">
-			Zuletzt hinzugefügt
+			{m.dashboard_recent_heading()}
 		</h2>
 		{#each recentDocs as doc (doc.id)}
 			<div class="flex items-center justify-between border-b border-line py-2 last:border-0">