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/messages/es.json

@@ -907,5 +907,68 @@
 	"bulk_edit_loading": "Cargando documentos…",
 	"bulk_edit_all_x_failed": "No se pudieron cargar los resultados del filtro; vuelve a intentarlo.",
 	"bulk_edit_topbar_title": "Edición masiva",
-	"bulk_edit_count_pill": "Se editarán {count}"
+	"bulk_edit_count_pill": "Se editarán {count}",
+
+	"nav_stammbaum": "Árbol genealógico",
+
+	"error_relationship_not_found": "La relación no fue encontrada.",
+	"error_circular_relationship": "Esta relación crearía un ciclo.",
+	"error_duplicate_relationship": "Esta relación ya existe.",
+
+	"relation_parent_of": "Progenitor de",
+	"relation_spouse_of": "Cónyuge",
+	"relation_sibling_of": "Hermano/a",
+	"relation_friend": "Amigo/a",
+	"relation_colleague": "Colega",
+	"relation_employer": "Empleador",
+	"relation_doctor": "Médico",
+	"relation_neighbor": "Vecino/a",
+	"relation_other": "Otro",
+
+	"relation_inferred_parent": "Progenitor",
+	"relation_inferred_child": "Hijo/a",
+	"relation_inferred_spouse": "Cónyuge",
+	"relation_inferred_sibling": "Hermano/a",
+	"relation_inferred_grandparent": "Abuelo/a",
+	"relation_inferred_grandchild": "Nieto/a",
+	"relation_inferred_great_grandparent": "Bisabuelo/a",
+	"relation_inferred_great_grandchild": "Bisnieto/a",
+	"relation_inferred_uncle_aunt": "Tío/Tía",
+	"relation_inferred_niece_nephew": "Sobrino/a",
+	"relation_inferred_great_uncle_aunt": "Tío/a abuelo/a",
+	"relation_inferred_great_niece_nephew": "Sobrino/a nieto/a",
+	"relation_inferred_inlaw_parent": "Suegro/a",
+	"relation_inferred_inlaw_child": "Yerno/Nuera",
+	"relation_inferred_sibling_inlaw": "Cuñado/a",
+	"relation_inferred_cousin_1": "Primo/a",
+	"relation_inferred_distant": "Pariente lejano",
+
+	"doc_details_field_relationship": "Parentesco",
+
+	"stammbaum_empty_heading": "Aún no hay miembros de la familia",
+	"stammbaum_empty_body": "Marca a una persona como miembro de la familia en su página de edición para que aparezca aquí.",
+	"stammbaum_empty_link": "→ Ir a la lista de personas",
+	"stammbaum_panel_direct_rels": "Relaciones directas",
+	"stammbaum_panel_derived_rels": "Relaciones derivadas",
+	"stammbaum_panel_to_person": "Ir a la persona →",
+	"stammbaum_panel_add_rel": "+ Añadir relación",
+	"stammbaum_zoom_in": "Acercar",
+	"stammbaum_zoom_out": "Alejar",
+	"stammbaum_generations": "Generaciones",
+
+	"relation_error_duplicate": "Esta relación ya existe.",
+	"relation_error_circular": "Esta relación crearía un ciclo.",
+	"relation_error_self": "Una persona no puede estar relacionada consigo misma.",
+	"relation_year_error_bis_before_von": "El año final no puede ser anterior al año inicial.",
+	"relation_label_family_member": "Miembro de la familia",
+	"relation_label_in_tree": "Aparece en el árbol genealógico",
+	"relation_label_view_in_tree": "Ver →",
+	"relation_label_direct": "Relaciones directas",
+	"relation_label_derived": "Relaciones derivadas",
+	"relation_btn_add": "Añadir",
+	"relation_btn_save": "Guardar",
+	"relation_btn_cancel": "Cancelar",
+
+	"person_relationships_heading": "Relaciones",
+	"person_relationships_empty": "Aún no se conocen relaciones."
 }