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/conversations/ConversationTimeline.svelte

@@ -15,7 +15,12 @@ let {
 		documentDate?: string;
 		location?: string;
 		status: string;
-		sender?: { id: string; firstName: string; lastName: string } | null;
+		sender?: {
+			id: string;
+			firstName?: string | null;
+			lastName: string;
+			displayName: string;
+		} | null;
 	}[];
 	senderId: string;
 	receiverId: string;
@@ -106,7 +111,7 @@ const enrichedDocuments = $derived(
 									: 'border-line bg-surface text-ink'}"
 							>
 								{#if doc.sender}
-									{doc.sender.firstName[0]}{doc.sender.lastName[0]}
+									{doc.sender.firstName ? doc.sender.firstName[0] : doc.sender.lastName[0]}{doc.sender.lastName[0]}
 								{:else}
 									?
 								{/if}