refactor(stammbaum): extract chipLabel/otherName to shared relationshipLabels helper

Addresses @felix blocker: both functions were duplicated verbatim in
StammbaumCard.svelte and StammbaumSidePanel.svelte. Now exported from
$lib/relationshipLabels.ts with perspectivePersonId as an explicit param.
8 unit tests added (red→green).

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

frontend/src/lib/relationshipLabels.ts

@@ -1,4 +1,37 @@
 import * as m from '$lib/paraglide/messages.js';
+import type { components } from '$lib/generated/api';
+
+type RelationshipDTO = components['schemas']['RelationshipDTO'];
+
+export function chipLabel(rel: RelationshipDTO, perspectivePersonId: string): string {
+	const viewpointIsSubject = rel.personId === perspectivePersonId;
+	switch (rel.relationType) {
+		case 'PARENT_OF':
+			return viewpointIsSubject ? m.relation_parent_of() : m.relation_child_of();
+		case 'SPOUSE_OF':
+			return m.relation_spouse_of();
+		case 'SIBLING_OF':
+			return m.relation_sibling_of();
+		case 'FRIEND':
+			return m.relation_friend();
+		case 'COLLEAGUE':
+			return m.relation_colleague();
+		case 'EMPLOYER':
+			return m.relation_employer();
+		case 'DOCTOR':
+			return m.relation_doctor();
+		case 'NEIGHBOR':
+			return m.relation_neighbor();
+		default:
+			return m.relation_other();
+	}
+}
+
+export function otherName(rel: RelationshipDTO, perspectivePersonId: string): string {
+	return rel.personId === perspectivePersonId
+		? rel.relatedPersonDisplayName
+		: rel.personDisplayName;
+}
 
 /**
  * Maps a backend inferred-label key (parent, uncle_aunt, ...) to its