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/CLAUDE.md

@@ -53,7 +53,6 @@ src/
 │   ├── geschichte/      # Geschichte (story) domain: editor + card
 │   ├── notification/    # Notification bell + dropdown + store
 │   ├── activity/        # Activity feed (Chronik) components
-│   ├── conversation/    # Bilateral conversation (Briefwechsel) components
 │   ├── ocr/             # OCR progress, training cards, trigger
 │   ├── user/            # User profile/password/groups section components
 │   ├── shared/          # Cross-domain utilities and primitives

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';