feat(person): ErrorCode.PERSON_RENAME_CONFLICT for optimistic-lock conflicts

Adds the structured error code returned when a rename rolls back because a
referenced transcription block was edited concurrently (OptimisticLockException
on transcription_blocks.version). Mirrors the contract in
frontend src/lib/errors.ts and adds the localised message keys
error_person_rename_conflict in de/en/es so the UI surfaces a retry hint
instead of a generic 500.

The actual translation of OptimisticLockException → DomainException
(PERSON_RENAME_CONFLICT) lands in the next commit alongside the integration
test that proves the rollback semantics.

Refs #362

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/errors.ts

@@ -8,6 +8,7 @@ export type ErrorCode =
 	| 'PERSON_NOT_FOUND'
 	| 'ALIAS_NOT_FOUND'
 	| 'INVALID_PERSON_TYPE'
+	| 'PERSON_RENAME_CONFLICT'
 	| 'DOCUMENT_NOT_FOUND'
 	| 'DOCUMENT_NO_FILE'
 	| 'FILE_NOT_FOUND'
@@ -79,6 +80,8 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_alias_not_found();
 		case 'INVALID_PERSON_TYPE':
 			return m.error_invalid_person_type();
+		case 'PERSON_RENAME_CONFLICT':
+			return m.error_person_rename_conflict();
 		case 'DOCUMENT_NOT_FOUND':
 			return m.error_document_not_found();
 		case 'DOCUMENT_NO_FILE':