refactor(conversations): use GroupDivider in ConversationTimeline

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

frontend/src/routes/conversations/ConversationTimeline.svelte

@@ -1,6 +1,7 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
 import { formatDate } from '$lib/utils/date';
+import GroupDivider from '$lib/components/GroupDivider.svelte';
 
 let {
 	documents,
@@ -84,13 +85,7 @@ const enrichedDocuments = $derived(
 		<div class="relative z-10 flex flex-col gap-4">
 			{#each enrichedDocuments as { doc, year, showYearDivider } (doc.id)}
 				{#if showYearDivider}
-					<div data-testid="year-divider" class="relative flex items-center py-2 text-center">
-						<div class="flex-grow border-t border-line"></div>
-						<span class="mx-4 font-sans text-xs font-bold tracking-widest text-ink/40 uppercase"
-							>{year}</span
-						>
-						<div class="flex-grow border-t border-line"></div>
-					</div>
+					<GroupDivider label={String(year)} />
 				{/if}
 				{@const isRight = doc.sender?.id === senderId}