docs: align ErrorCode 4-step checklist in CLAUDE.md; note frontend sync in ARCHITECTURE.md

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

docs/ARCHITECTURE.md

@@ -63,7 +63,7 @@ Members of the cross-cutting layer have no entity of their own, no user-facing C
 | `audit` | Append-only event store (`audit_log`) for all domain mutations. Feeds the activity feed and Family Pulse dashboard. | Consumed by 5+ domains; no user-facing CRUD of its own |
 | `config` | Infrastructure bean definitions: `MinioConfig`, `AsyncConfig`, `WebConfig` | Framework infra; no business logic |
 | `dashboard` | Stats aggregation for the admin dashboard and Family Pulse widget | Aggregates from 3+ domains; no owned entities |
-| `exception` | `DomainException`, `ErrorCode` enum, `GlobalExceptionHandler` | Framework infra; consumed by every controller and service |
+| `exception` | `DomainException`, `ErrorCode` enum, `GlobalExceptionHandler` | Framework infra; consumed by every controller and service. Adding a new `ErrorCode` requires matching updates in `frontend/src/lib/shared/errors.ts` and all three `messages/*.json` locale files. |
 | `filestorage` | `FileService` — MinIO/S3 upload, download, presigned-URL generation | Generic service; consumed by `document` and `ocr` |
 | `importing` | `MassImportService` — async ODS/Excel batch import | Orchestrates across `person`, `tag`, `document` |
 | `security` | `SecurityConfig`, `Permission` enum, `@RequirePermission` annotation, `PermissionAspect` (AOP) | Framework infra; enforced globally across all controllers |