feat(persons): add sort toggle to person document list (issue #24)

Extracted sortDocumentsByDate utility with full Vitest coverage (6 tests),
wired it into the person detail page with a DESC/ASC toggle button, and
added an E2E smoke test for the toggle interaction.

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

frontend/src/lib/utils/sort.spec.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+import { sortDocumentsByDate } from './sort';
+
+const doc = (id: string, documentDate: string | null) =>
+	({ id, documentDate } as { id: string; documentDate: string | null });
+
+describe('sortDocumentsByDate', () => {
+	it('sorts DESC by default — newest first', () => {
+		const docs = [doc('a', '1920-01-01'), doc('b', '1950-06-15'), doc('c', '1935-03-10')];
+		const result = sortDocumentsByDate(docs, 'DESC');
+		expect(result.map((d) => d.id)).toEqual(['b', 'c', 'a']);
+	});
+
+	it('sorts ASC — oldest first', () => {
+		const docs = [doc('a', '1920-01-01'), doc('b', '1950-06-15'), doc('c', '1935-03-10')];
+		const result = sortDocumentsByDate(docs, 'ASC');
+		expect(result.map((d) => d.id)).toEqual(['a', 'c', 'b']);
+	});
+
+	it('places documents without a date last in DESC', () => {
+		const docs = [doc('a', null), doc('b', '1940-01-01'), doc('c', null)];
+		const result = sortDocumentsByDate(docs, 'DESC');
+		expect(result[0].id).toBe('b');
+		expect(result.slice(1).map((d) => d.id)).toContain('a');
+		expect(result.slice(1).map((d) => d.id)).toContain('c');
+	});
+
+	it('places documents without a date last in ASC', () => {
+		const docs = [doc('a', null), doc('b', '1940-01-01'), doc('c', null)];
+		const result = sortDocumentsByDate(docs, 'ASC');
+		expect(result[0].id).toBe('b');
+	});
+
+	it('does not mutate the original array', () => {
+		const docs = [doc('a', '1950-01-01'), doc('b', '1920-01-01')];
+		const original = [...docs];
+		sortDocumentsByDate(docs, 'ASC');
+		expect(docs).toEqual(original);
+	});
+
+	it('returns an empty array unchanged', () => {
+		expect(sortDocumentsByDate([], 'DESC')).toEqual([]);
+	});
+});

frontend/src/lib/utils/sort.ts

@@ -0,0 +1,19 @@
+export type SortDir = 'ASC' | 'DESC';
+
+/**
+ * Returns a new array of documents sorted by documentDate.
+ * Documents without a date are always placed last, regardless of direction.
+ */
+export function sortDocumentsByDate<T extends { documentDate?: string | null }>(
+	docs: T[],
+	dir: SortDir
+): T[] {
+	return [...docs].sort((a, b) => {
+		const da = a.documentDate ?? '';
+		const db = b.documentDate ?? '';
+		if (!da && !db) return 0;
+		if (!da) return 1;
+		if (!db) return -1;
+		return dir === 'DESC' ? db.localeCompare(da) : da.localeCompare(db);
+	});
+}