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/routes/persons/[id]/+page.svelte

@@ -72,14 +72,12 @@ const coCorrespondents = $derived.by(() => {
 				documents={sentDocuments}
 				heading={m.person_docs_heading()}
 				emptyMessage={m.person_no_docs()}
-				variant="sent"
 			/>
 
 			<PersonDocumentList
 				documents={receivedDocuments}
 				heading={m.person_received_docs_heading()}
 				emptyMessage={m.person_no_received_docs()}
-				variant="received"
 			/>
 		</div>
 	</div>