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/DashboardMentions.svelte

@@ -1,4 +1,6 @@
 <script lang="ts">
+import * as m from '$lib/paraglide/messages.js';
+
 type NotificationDTO = {
 	id: string;
 	type: 'REPLY' | 'MENTION';
@@ -18,7 +20,7 @@ let { mentions }: Props = $props();
 {#if mentions.length > 0}
 	<div data-testid="dashboard-mentions" 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">
-			Benachrichtigungen
+			{m.dashboard_notifications_heading()}
 		</h2>
 		{#each mentions as mention (mention.id)}
 			<div class="flex items-center gap-3 border-b border-line py-2 last:border-0">
@@ -29,8 +31,10 @@ let { mentions }: Props = $props();
 					>
 						{mention.actorName ?? ''}
 						{#if mention.type === 'MENTION'}<span class="ml-1 font-sans text-xs text-gray-400"
-								>erwähnt Sie</span
-							>{:else}<span class="ml-1 font-sans text-xs text-gray-400">hat geantwortet</span>{/if}
+								>{m.dashboard_notification_mentioned()}</span
+							>{:else}<span class="ml-1 font-sans text-xs text-gray-400"
+								>{m.dashboard_notification_replied()}</span
+							>{/if}
 					</a>
 				{:else}
 					<span class="font-serif text-sm text-ink">{mention.actorName ?? ''}</span>