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.test.ts

@@ -0,0 +1,66 @@
+import { describe, expect, it } from 'vitest';
+import { m } from '$lib/paraglide/messages.js';
+import { chipLabel, otherName } from './relationshipLabels';
+import type { components } from '$lib/generated/api';
+
+type RelationshipDTO = components['schemas']['RelationshipDTO'];
+
+const ALICE_ID = 'alice-uuid';
+const BOB_ID = 'bob-uuid';
+
+function makeRel(
+	relationType: RelationshipDTO['relationType'],
+	override: Partial<RelationshipDTO> = {}
+): RelationshipDTO {
+	return {
+		id: 'rel-1',
+		personId: ALICE_ID,
+		relatedPersonId: BOB_ID,
+		personDisplayName: 'Alice',
+		relatedPersonDisplayName: 'Bob',
+		relationType,
+		...override
+	};
+}
+
+describe('chipLabel', () => {
+	it('returns parent_of when perspective is the subject of PARENT_OF', () => {
+		const rel = makeRel('PARENT_OF');
+		expect(chipLabel(rel, ALICE_ID)).toBe(m.relation_parent_of());
+	});
+
+	it('returns child_of when perspective is the object of PARENT_OF', () => {
+		const rel = makeRel('PARENT_OF');
+		expect(chipLabel(rel, BOB_ID)).toBe(m.relation_child_of());
+	});
+
+	it('returns spouse_of for SPOUSE_OF regardless of perspective', () => {
+		const rel = makeRel('SPOUSE_OF');
+		expect(chipLabel(rel, ALICE_ID)).toBe(m.relation_spouse_of());
+		expect(chipLabel(rel, BOB_ID)).toBe(m.relation_spouse_of());
+	});
+
+	it('returns sibling_of for SIBLING_OF', () => {
+		expect(chipLabel(makeRel('SIBLING_OF'), ALICE_ID)).toBe(m.relation_sibling_of());
+	});
+
+	it('returns friend for FRIEND', () => {
+		expect(chipLabel(makeRel('FRIEND'), ALICE_ID)).toBe(m.relation_friend());
+	});
+
+	it('returns other for OTHER', () => {
+		expect(chipLabel(makeRel('OTHER'), ALICE_ID)).toBe(m.relation_other());
+	});
+});
+
+describe('otherName', () => {
+	it('returns relatedPersonDisplayName when perspective is the subject', () => {
+		const rel = makeRel('PARENT_OF');
+		expect(otherName(rel, ALICE_ID)).toBe('Bob');
+	});
+
+	it('returns personDisplayName when perspective is the object', () => {
+		const rel = makeRel('PARENT_OF');
+		expect(otherName(rel, BOB_ID)).toBe('Alice');
+	});
+});