feat(#145): add DashboardMentions widget component

Shows unread mention notifications as a dashboard widget. Renders
nothing when the mentions list is empty.

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

frontend/src/lib/components/DashboardMentions.svelte

@@ -0,0 +1,38 @@
+<script lang="ts">
+type NotificationDTO = {
+	id: string;
+	type: 'REPLY' | 'MENTION';
+	documentId?: string;
+	read: boolean;
+	createdAt: string;
+	actorName?: string;
+};
+
+interface Props {
+	mentions: NotificationDTO[];
+}
+
+let { mentions }: Props = $props();
+</script>
+
+{#if mentions.length > 0}
+	<div data-testid="dashboard-mentions" class="border-brand-sand rounded-sm border bg-white p-6">
+		<h2 class="mb-4 font-sans text-xs font-bold tracking-widest text-gray-400 uppercase">
+			Erwähnungen
+		</h2>
+		{#each mentions as mention (mention.id)}
+			<div class="border-brand-sand flex items-center gap-3 border-b py-2 last:border-0">
+				{#if mention.documentId}
+					<a
+						href="/documents/{mention.documentId}"
+						class="font-serif text-sm text-ink hover:text-brand-navy"
+					>
+						{mention.actorName ?? ''}
+					</a>
+				{:else}
+					<span class="font-serif text-sm text-ink">{mention.actorName ?? ''}</span>
+				{/if}
+			</div>
+		{/each}
+	</div>
+{/if}