feat(stammbaum): add i18n keys (de/en/es) + mirror error codes

In each of de/en/es:
- nav_stammbaum
- 9 relation_<type>_of keys for the stored relation types
- 17 relation_inferred_<label> keys covering everything LABEL_MAP emits
  (parent/child/spouse/sibling, grand*, great-grand*, uncle/aunt,
  niece/nephew, in-laws, cousin, distant)
- doc_details_field_relationship — badge label "Verwandtschaft"
- stammbaum_empty_*, stammbaum_panel_*, stammbaum_zoom_*,
  stammbaum_generations
- relation_error_* (inline form errors), relation_year_error_*,
  relation_label_*, relation_btn_*
- person_relationships_heading + person_relationships_empty
- error_relationship_not_found / error_circular_relationship /
  error_duplicate_relationship for the centralised error mapper

frontend/src/lib/errors.ts mirrors the backend's three new ErrorCodes
and routes them through getErrorMessage().

Refs #358.

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

frontend/src/lib/errors.ts

@@ -38,6 +38,9 @@ export type ErrorCode =
 	| 'TAG_NOT_FOUND'
 	| 'TAG_MERGE_SELF'
 	| 'TAG_MERGE_INVALID_TARGET'
+	| 'RELATIONSHIP_NOT_FOUND'
+	| 'CIRCULAR_RELATIONSHIP'
+	| 'DUPLICATE_RELATIONSHIP'
 	| 'MISSING_CREDENTIALS'
 	| 'UNAUTHORIZED'
 	| 'FORBIDDEN'
@@ -136,6 +139,12 @@ export function getErrorMessage(code: ErrorCode | string | undefined): string {
 			return m.error_tag_merge_self();
 		case 'TAG_MERGE_INVALID_TARGET':
 			return m.error_tag_merge_invalid_target();
+		case 'RELATIONSHIP_NOT_FOUND':
+			return m.error_relationship_not_found();
+		case 'CIRCULAR_RELATIONSHIP':
+			return m.error_circular_relationship();
+		case 'DUPLICATE_RELATIONSHIP':
+			return m.error_duplicate_relationship();
 		case 'MISSING_CREDENTIALS':
 			return m.login_error_missing_credentials();
 		case 'UNAUTHORIZED':