feat(persons): add formatLifeDateRange + formatDocumentStatus utility functions

Unit tests for both; i18n keys for doc status and person stats bar;
PERSON_NOT_FOUND added to frontend ErrorCode type.

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

frontend/src/lib/utils/documentStatusLabel.ts

@@ -0,0 +1,22 @@
+import { m } from '$lib/paraglide/messages.js';
+
+/**
+ * Maps a document status string to a localised human-readable label.
+ * Falls back to "Unknown" for unrecognised values.
+ */
+export function formatDocumentStatus(status: string): string {
+	switch (status) {
+		case 'PLACEHOLDER':
+			return m.doc_status_placeholder();
+		case 'UPLOADED':
+			return m.doc_status_uploaded();
+		case 'TRANSCRIBED':
+			return m.doc_status_transcribed();
+		case 'REVIEWED':
+			return m.doc_status_reviewed();
+		case 'ARCHIVED':
+			return m.doc_status_archived();
+		default:
+			return m.doc_status_unknown();
+	}
+}