feat(frontend): single-person mode in +page.server.ts load function

Loads documents whenever senderId is set, using the optional receiverId
param to switch between single-person and bilateral query modes.

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

frontend/src/routes/korrespondenz/+page.server.ts

@@ -16,14 +16,14 @@ export async function load({ url, fetch }) {
 
 	const requests: Promise<void>[] = [];
 
-	if (senderId && receiverId) {
+	if (senderId) {
 		requests.push(
 			api
 				.GET('/api/documents/conversation', {
 					params: {
 						query: {
 							senderId,
-							receiverId,
+							receiverId: receiverId || undefined,
 							dir,
 							from: from || undefined,
 							to: to || undefined
@@ -34,9 +34,7 @@ export async function load({ url, fetch }) {
 					documents = data ?? [];
 				})
 		);
-	}
 
-	if (senderId) {
 		requests.push(
 			api.GET('/api/persons/{id}', { params: { path: { id: senderId } } }).then(({ data }) => {
 				const p = data as { firstName: string; lastName: string } | undefined;