fix(briefwechsel): repair 500 by consuming backend thumbnailUrl directly

ConversationThumbnail still imported the `$lib/thumbnails` helper that
a02f6cdc deleted, so every SSR render of /briefwechsel crashed with
"Cannot find module '$lib/thumbnails'". Finish that refactor by reading
`doc.thumbnailUrl` straight off the Document DTO (same shape
DocumentThumbnail already uses), and update the spec fixtures to match.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/components/ConversationThumbnail.svelte

@@ -1,17 +1,14 @@
 <script lang="ts">
-import { thumbnailUrl } from '$lib/thumbnails';
+import type { components } from '$lib/generated/api';
 
-type Doc = {
-	id: string;
-	thumbnailKey?: string;
-	thumbnailGeneratedAt?: string;
-	thumbnailAspect?: 'PORTRAIT' | 'LANDSCAPE';
-	pageCount?: number;
-};
+type Doc = Pick<
+	components['schemas']['Document'],
+	'id' | 'thumbnailUrl' | 'thumbnailAspect' | 'pageCount'
+>;
 
 let { doc }: { doc: Doc } = $props();
 
-const url = $derived(thumbnailUrl(doc));
+const url = $derived(doc.thumbnailUrl ?? null);
 const aspect = $derived(doc.thumbnailAspect ?? 'PORTRAIT');
 const pageCount = $derived(doc.pageCount ?? 1);
 const tileClass = $derived(aspect === 'LANDSCAPE' ? 'h-[120px] w-[168px]' : 'h-[168px] w-[120px]');