feat(frontend): render DocumentThumbnail in DocumentRow and PersonDocumentList

Home search rows and person detail sidebars now show the real
first-page preview when one exists, falling back to the PDF icon
for documents the backfill hasn't processed yet. The old `variant`
prop on PersonDocumentList is removed — it tinted the icon
differently for sent vs received, which no longer applies with a
uniform thumbnail tile.

Refs #307

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

frontend/src/lib/components/DocumentRow.svelte

@@ -6,6 +6,7 @@ import { formatDate } from '$lib/utils/date';
 import * as m from '$lib/paraglide/messages.js';
 import ProgressRing from './ProgressRing.svelte';
 import ContributorStack from './ContributorStack.svelte';
+import DocumentThumbnail from './DocumentThumbnail.svelte';
 
 type DocumentSearchItem = components['schemas']['DocumentSearchItem'];
 
@@ -37,7 +38,10 @@ function safeTagColor(color: string | null | undefined): string {
 
 <li class="group transition-colors duration-200 hover:bg-muted/50">
 	<a href="/documents/{doc.id}" class="block px-4 py-4 sm:py-5">
-		<div class="flex gap-0 sm:gap-5">
+		<div class="flex gap-3 sm:gap-5">
+			<!-- Thumbnail tile -->
+			<DocumentThumbnail doc={doc} />
+
 			<!-- Left column -->
 			<div class="flex-1 sm:border-r sm:border-line sm:pr-5">
 				<!-- Title -->