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.spec.ts

@@ -0,0 +1,28 @@
+import { describe, it, expect } from 'vitest';
+import { formatDocumentStatus } from './documentStatusLabel';
+
+describe('formatDocumentStatus', () => {
+	it('maps PLACEHOLDER to correct label', () => {
+		expect(formatDocumentStatus('PLACEHOLDER')).toBe('Platzhalter');
+	});
+
+	it('maps UPLOADED to correct label', () => {
+		expect(formatDocumentStatus('UPLOADED')).toBe('Hochgeladen');
+	});
+
+	it('maps TRANSCRIBED to correct label', () => {
+		expect(formatDocumentStatus('TRANSCRIBED')).toBe('Transkribiert');
+	});
+
+	it('maps REVIEWED to correct label', () => {
+		expect(formatDocumentStatus('REVIEWED')).toBe('Geprüft');
+	});
+
+	it('maps ARCHIVED to correct label', () => {
+		expect(formatDocumentStatus('ARCHIVED')).toBe('Archiviert');
+	});
+
+	it('returns fallback for unknown status', () => {
+		expect(formatDocumentStatus('SOMETHING_NEW')).toBe('Unbekannt');
+	});
+});