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/errors.ts

@@ -5,6 +5,7 @@ import * as m from '$lib/paraglide/messages.js';
  * Keep in sync with backend/src/main/java/org/raddatz/familienarchiv/exception/ErrorCode.java
  */
 export type ErrorCode =
+	| 'PERSON_NOT_FOUND'
 	| 'DOCUMENT_NOT_FOUND'
 	| 'DOCUMENT_NO_FILE'
 	| 'FILE_NOT_FOUND'
@@ -47,6 +48,8 @@ export async function parseBackendError(res: Response): Promise<BackendError | n
 /** Returns a localised message for the given error code via Paraglide. Falls back to INTERNAL_ERROR. */
 export function getErrorMessage(code: ErrorCode | string | undefined): string {
 	switch (code) {
+		case 'PERSON_NOT_FOUND':
+			return m.error_person_not_found();
 		case 'DOCUMENT_NOT_FOUND':
 			return m.error_document_not_found();
 		case 'DOCUMENT_NO_FILE':