fix(ui): guard selectPerson against empty id

Restores early return when id is empty, preventing a wasteful
navigation to /briefwechsel with no senderId param.

Refs: #179

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

frontend/src/routes/briefwechsel/+page.svelte

@@ -86,6 +86,7 @@ function swapPersons() {
 }
 
 function selectPerson(id: string) {
+	if (!id) return;
 	senderId = id;
 	receiverId = '';
 	applyFilters();