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/en.json

@@ -907,5 +907,68 @@
 	"bulk_edit_loading": "Loading documents…",
 	"bulk_edit_all_x_failed": "Could not load filter results — please retry.",
 	"bulk_edit_topbar_title": "Bulk edit",
-	"bulk_edit_count_pill": "{count} will be edited"
+	"bulk_edit_count_pill": "{count} will be edited",
+
+	"nav_stammbaum": "Family tree",
+
+	"error_relationship_not_found": "Relationship not found.",
+	"error_circular_relationship": "This relationship would form a cycle.",
+	"error_duplicate_relationship": "This relationship already exists.",
+
+	"relation_parent_of": "Parent of",
+	"relation_spouse_of": "Spouse",
+	"relation_sibling_of": "Sibling",
+	"relation_friend": "Friend",
+	"relation_colleague": "Colleague",
+	"relation_employer": "Employer",
+	"relation_doctor": "Doctor",
+	"relation_neighbor": "Neighbour",
+	"relation_other": "Other",
+
+	"relation_inferred_parent": "Parent",
+	"relation_inferred_child": "Child",
+	"relation_inferred_spouse": "Spouse",
+	"relation_inferred_sibling": "Sibling",
+	"relation_inferred_grandparent": "Grandparent",
+	"relation_inferred_grandchild": "Grandchild",
+	"relation_inferred_great_grandparent": "Great-grandparent",
+	"relation_inferred_great_grandchild": "Great-grandchild",
+	"relation_inferred_uncle_aunt": "Uncle/Aunt",
+	"relation_inferred_niece_nephew": "Niece/Nephew",
+	"relation_inferred_great_uncle_aunt": "Great-uncle/Aunt",
+	"relation_inferred_great_niece_nephew": "Great-niece/Nephew",
+	"relation_inferred_inlaw_parent": "Parent-in-law",
+	"relation_inferred_inlaw_child": "Child-in-law",
+	"relation_inferred_sibling_inlaw": "Sibling-in-law",
+	"relation_inferred_cousin_1": "Cousin",
+	"relation_inferred_distant": "Distant relative",
+
+	"doc_details_field_relationship": "Relationship",
+
+	"stammbaum_empty_heading": "No family members yet",
+	"stammbaum_empty_body": "Mark a person as a family member on their edit page so they appear here.",
+	"stammbaum_empty_link": "→ Go to person list",
+	"stammbaum_panel_direct_rels": "Direct relationships",
+	"stammbaum_panel_derived_rels": "Derived relationships",
+	"stammbaum_panel_to_person": "Go to person page →",
+	"stammbaum_panel_add_rel": "+ Add relationship",
+	"stammbaum_zoom_in": "Zoom in",
+	"stammbaum_zoom_out": "Zoom out",
+	"stammbaum_generations": "Generations",
+
+	"relation_error_duplicate": "This relationship already exists.",
+	"relation_error_circular": "This relationship would form a cycle.",
+	"relation_error_self": "A person cannot be related to themselves.",
+	"relation_year_error_bis_before_von": "End year must not precede start year.",
+	"relation_label_family_member": "Family member",
+	"relation_label_in_tree": "Appears in the family tree",
+	"relation_label_view_in_tree": "View →",
+	"relation_label_direct": "Direct relationships",
+	"relation_label_derived": "Derived relationships",
+	"relation_btn_add": "Add",
+	"relation_btn_save": "Save",
+	"relation_btn_cancel": "Cancel",
+
+	"person_relationships_heading": "Relationships",
+	"person_relationships_empty": "No relationships known yet."
 }