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/DashboardNeedsMetadata.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+
 type IncompleteDocumentDTO = {
 	id: string;
 	title: string;
@@ -14,7 +16,7 @@ let { incompleteDocs }: Props = $props();
 {#if incompleteDocs.length > 0}
 	<div data-testid="dashboard-needs-metadata" 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">
-			Metadaten fehlen
+			{m.dashboard_needs_metadata_heading()}
 		</h2>
 		{#each incompleteDocs as doc (doc.id)}
 			<div class="flex items-center border-b border-line py-2 last:border-0">
@@ -27,7 +29,9 @@ let { incompleteDocs }: Props = $props();
 			</div>
 		{/each}
 		<div class="mt-4">
-			<a href="/enrich" class="font-sans text-xs text-ink-2 hover:text-ink"> Alle anzeigen </a>
+			<a href="/enrich" class="font-sans text-xs text-ink-2 hover:text-ink"
+				>{m.dashboard_needs_metadata_show_all()}</a
+			>
 		</div>
 	</div>
 {/if}