feat(frontend): replace all name concatenation with displayName

- Add displayName default method to PersonSummaryDTO
- Update native SQL queries to include title, person_type columns
- Add getInitials() utility to personFormat.ts
- Update abbreviateName/abbreviateCompact for nullable firstName
- Replace firstName+lastName concatenation with displayName in all
  person-displaying components and server load files
- Regenerate API types with displayName on Person and PersonSummaryDTO

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

frontend/src/routes/briefwechsel/ConversationTimeline.svelte

@@ -10,8 +10,13 @@ interface Props {
 		documentDate?: string;
 		location?: string;
 		status: string;
-		sender?: { id: string; firstName: string; lastName: string } | null;
-		receivers?: { id: string; firstName: string; lastName: string }[];
+		sender?: {
+			id: string;
+			firstName?: string | null;
+			lastName: string;
+			displayName: string;
+		} | null;
+		receivers?: { id: string; firstName?: string | null; lastName: string; displayName: string }[];
 	}[];
 	senderId: string;
 	receiverId?: string;
@@ -67,9 +72,9 @@ function statusDotClass(status: string): string {
 function otherPartyName(doc: (typeof documents)[number]): string {
 	if (doc.sender?.id === senderId) {
 		const r = doc.receivers?.[0];
-		return r ? `${r.firstName} ${r.lastName}` : m.conv_no_party();
+		return r ? r.displayName : m.conv_no_party();
 	}
-	return doc.sender ? `${doc.sender.firstName} ${doc.sender.lastName}` : m.conv_no_party();
+	return doc.sender ? doc.sender.displayName : m.conv_no_party();
 }
 
 const newDocUrl = $derived(