refactor: move conversation domain to lib/conversation/

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

frontend/src/lib/components/ThumbnailRow.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-import ConversationThumbnail from '$lib/components/ConversationThumbnail.svelte';
+import ConversationThumbnail from '$lib/conversation/ConversationThumbnail.svelte';
 import TagChipList from '$lib/components/TagChipList.svelte';
 import { formatDate } from '$lib/utils/date';
 import * as m from '$lib/paraglide/messages.js';