refactor(documents): rename year-card testid to group-card

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

frontend/src/routes/DocumentList.svelte

@@ -103,7 +103,7 @@ function groupByReceiver(docItems: DocumentSearchItem[]) {
 	<!-- GROUP CARDS -->
 	{#each groups as group (group.label)}
 		<div
-			data-testid="year-card"
+			data-testid="group-card"
 			class="mb-4 overflow-hidden border border-line bg-surface shadow-sm"
 		>
 			<div class="border-b border-line bg-muted px-5 py-2">