refactor(document): move ConversationThumbnail into lib/document

With the Briefwechsel view gone, lib/conversation/ held a single shared
component whose only consumer is lib/document/ThumbnailRow. Move it (and
its spec) into lib/document/, update the import, delete the now-empty
lib/conversation/ folder, and fix the stale frontend/CLAUDE.md lib map.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/document/ThumbnailRow.svelte

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