feat(relationship): add formatRelationshipDateRange helper

Plain-text "from – to" range formatter for relationship dates, delegating all
precision rendering to the shared formatDocumentDate (zero new precision logic).
Lives in $lib/person next to personLifeDates and reuses the existing
person → shared boundary. From-only renders just the start (no trailing dash),
no dates renders nothing.

REQ-015

Refs #837
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/relationshipDates.spec.ts

@@ -0,0 +1,65 @@
+import { describe, expect, it } from 'vitest';
+import { formatRelationshipDateRange } from './relationshipDates';
+
+// Delegates all precision rendering to formatDocumentDate — these tests pin the
+// composition (dash, single sides, empty state) and one rendering per precision,
+// plus en/es for DAY/MONTH so a German-month leak is caught here, not on a card.
+describe('formatRelationshipDateRange', () => {
+	describe('both dates (de default)', () => {
+		it('renders DAY precision as full dates', () => {
+			expect(formatRelationshipDateRange('1923-05-12', 'DAY', '1958-06-13', 'DAY')).toBe(
+				'12. Mai 1923 – 13. Juni 1958'
+			);
+		});
+
+		it('renders MONTH precision as month + year', () => {
+			expect(formatRelationshipDateRange('1923-05-01', 'MONTH', '1958-06-01', 'MONTH')).toBe(
+				'Mai 1923 – Juni 1958'
+			);
+		});
+
+		it('renders YEAR precision as bare years', () => {
+			expect(formatRelationshipDateRange('1923-01-01', 'YEAR', '1958-01-01', 'YEAR')).toBe(
+				'1923 – 1958'
+			);
+		});
+
+		it('renders mixed precisions per side', () => {
+			expect(formatRelationshipDateRange('1923-05-12', 'DAY', '1958-01-01', 'YEAR')).toBe(
+				'12. Mai 1923 – 1958'
+			);
+		});
+	});
+
+	describe('single sides and empty states', () => {
+		it('renders from only without a trailing dash', () => {
+			expect(formatRelationshipDateRange('1923-05-12', 'DAY', null, null)).toBe('12. Mai 1923');
+		});
+
+		it('renders to only with a leading dash', () => {
+			expect(formatRelationshipDateRange(null, null, '1958-06-13', 'DAY')).toBe('– 13. Juni 1958');
+		});
+
+		it('renders nothing when both dates are missing (UNKNOWN)', () => {
+			expect(formatRelationshipDateRange(null, 'UNKNOWN', null, 'UNKNOWN')).toBe('');
+		});
+
+		it('renders nothing for a from-only with a null date', () => {
+			expect(formatRelationshipDateRange(null, null, null, null)).toBe('');
+		});
+	});
+
+	describe('localized months (catch German-month leak)', () => {
+		it('renders DAY in English with no German month name', () => {
+			const out = formatRelationshipDateRange('1923-05-12', 'DAY', null, null, 'en');
+			expect(out).toContain('May');
+			expect(out).not.toContain('Mai');
+			expect(out).toContain('1923');
+		});
+
+		it('renders MONTH in Spanish', () => {
+			const out = formatRelationshipDateRange('1923-05-01', 'MONTH', null, null, 'es');
+			expect(out.toLowerCase()).toContain('mayo');
+		});
+	});
+});

frontend/src/lib/person/relationshipDates.ts

@@ -0,0 +1,47 @@
+import { formatDocumentDate, type DatePrecision } from '$lib/shared/utils/documentDate';
+
+/**
+ * Formats one relationship endpoint (start or end) at the precision the data
+ * claims, delegating all rendering to {@link formatDocumentDate}. Returns '' for
+ * a missing date. A missing precision falls back to YEAR — pre-V78 rows only knew
+ * a year. Mirrors {@link formatLifeDate} for the person life-date pattern.
+ */
+function formatEnd(
+	date: string | null | undefined,
+	precision: DatePrecision | null | undefined,
+	locale?: string
+): string {
+	if (!date) {
+		return '';
+	}
+	return formatDocumentDate(date, precision ?? 'YEAR', null, null, locale);
+}
+
+/**
+ * Formats a relationship's start–end range as plain text, e.g. for a marriage row.
+ * Examples (de):
+ *   12. Mai 1923 – 13. Juni 1958   (both)
+ *   12. Mai 1923                   (start only — no trailing dash)
+ *   – 13. Juni 1958                (end only)
+ *   ""                             (neither — the caller renders no date line)
+ */
+export function formatRelationshipDateRange(
+	fromDate: string | null | undefined,
+	fromDatePrecision: DatePrecision | null | undefined,
+	toDate: string | null | undefined,
+	toDatePrecision: DatePrecision | null | undefined,
+	locale?: string
+): string {
+	const from = formatEnd(fromDate, fromDatePrecision, locale);
+	const to = formatEnd(toDate, toDatePrecision, locale);
+	if (from && to) {
+		return `${from} – ${to}`;
+	}
+	if (from) {
+		return from;
+	}
+	if (to) {
+		return `– ${to}`;
+	}
+	return '';
+}