fix(documents): suppress uppercase on person name group headers for SENDER/RECEIVER sort

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/routes/DocumentList.svelte

@@ -111,8 +111,10 @@ function groupByReceiver(docItems: DocumentSearchItem[]) {
 			<div class="border-b border-line bg-muted px-5 py-2">
 				<span
 					data-testid="group-header"
-					class="font-sans text-xs font-bold tracking-widest text-ink-3 uppercase"
-					>{group.label}</span
+					class="font-sans text-xs font-bold text-ink-3"
+					class:uppercase={sort !== 'SENDER' && sort !== 'RECEIVER'}
+					class:tracking-widest={sort !== 'SENDER' && sort !== 'RECEIVER'}
+					class:tracking-wide={sort === 'SENDER' || sort === 'RECEIVER'}>{group.label}</span
 				>
 			</div>
 			<ul class="divide-y divide-line">