test(korrespondenz): update and expand Vitest component specs

Update empty-state, swap-button, and new-doc-link tests to match redesigned
components. Add new tests for: single-person hint bar visibility, recent-persons
chips from localStorage, corrupt localStorage graceful handling, Row 2
aria-disabled state, and strip letter count in single-person and bilateral modes.

Fix CorrespondenzEmptyState to use {id, name} storage format matching
persistRecentPerson in +page.svelte.

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

frontend/src/routes/korrespondenz/CorrespondenzEmptyState.svelte

@@ -4,8 +4,7 @@ import { conv_empty_search_placeholder, conv_empty_recent_label } from '$lib/mes
 
 interface RecentPerson {
 	id: string;
-	firstName: string;
-	lastName: string;
+	name: string;
 }
 
 interface Props {
@@ -110,9 +109,9 @@ onMount(() => {
 							class="flex h-4 w-4 shrink-0 items-center justify-center rounded-full bg-[#002850] text-[10px] text-white"
 							aria-hidden="true"
 						>
-							{person.firstName[0]}{person.lastName[0]}
+							{person.name.charAt(0).toUpperCase()}
 						</span>
-						<span class="hidden sm:inline">{person.lastName}, </span>{person.firstName}
+						{person.name}
 					</button>
 				{/each}
 			</div>