feat(utils): add date-buckets helper for Chronik day grouping

Pure function bucketByDay(date, now?, locale?) returns one of
'today'|'yesterday'|'thisWeek'|'older' so ChronikTimeline can
bucket activity rows by relative day without pulling a date
library.

Handles:
- midnight boundary (startOfDay comparison)
- locale-aware week start (Monday for most locales, Sunday for en-US,
  en-CA, en-PH, ja-JP, he-IL, pt-BR)
- DST transitions (works off local calendar days)

Part of #285.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/lib/utils/date-buckets.spec.ts

@@ -0,0 +1,50 @@
+import { describe, it, expect } from 'vitest';
+import { bucketByDay } from './date-buckets';
+
+function date(iso: string): Date {
+	return new Date(iso);
+}
+
+describe('bucketByDay', () => {
+	// Wednesday 2026-04-22 at 12:00 Berlin. Week start (Mon) = 2026-04-20.
+	const now = date('2026-04-22T12:00:00+02:00');
+
+	it('returns "today" for a time earlier today', () => {
+		expect(bucketByDay(date('2026-04-22T06:00:00+02:00'), now, 'de-DE')).toBe('today');
+	});
+
+	it('returns "today" at exact midnight start of today', () => {
+		expect(bucketByDay(date('2026-04-22T00:00:00+02:00'), now, 'de-DE')).toBe('today');
+	});
+
+	it('returns "yesterday" for any time on the previous day', () => {
+		expect(bucketByDay(date('2026-04-21T23:59:59+02:00'), now, 'de-DE')).toBe('yesterday');
+		expect(bucketByDay(date('2026-04-21T00:00:00+02:00'), now, 'de-DE')).toBe('yesterday');
+	});
+
+	it('returns "thisWeek" for the Monday that starts this week (Monday-anchored, de-DE)', () => {
+		expect(bucketByDay(date('2026-04-20T10:00:00+02:00'), now, 'de-DE')).toBe('thisWeek');
+	});
+
+	it('returns "older" for anything before the start of this week (de-DE)', () => {
+		expect(bucketByDay(date('2026-04-19T23:00:00+02:00'), now, 'de-DE')).toBe('older');
+		expect(bucketByDay(date('2026-04-13T10:00:00+02:00'), now, 'de-DE')).toBe('older');
+	});
+
+	it('uses Sunday-start week for en-US', () => {
+		const sundayRef = date('2026-04-19T12:00:00+02:00');
+		expect(bucketByDay(date('2026-04-19T06:00:00+02:00'), sundayRef, 'en-US')).toBe('today');
+		expect(
+			bucketByDay(date('2026-04-13T10:00:00+02:00'), date('2026-04-18T12:00:00+02:00'), 'en-US')
+		).toBe('thisWeek');
+		expect(
+			bucketByDay(date('2026-04-11T10:00:00+02:00'), date('2026-04-18T12:00:00+02:00'), 'en-US')
+		).toBe('older');
+	});
+
+	it('handles DST spring-forward correctly (Europe/Berlin 2026-03-29)', () => {
+		const justAfterDst = date('2026-03-29T03:15:00+02:00');
+		const sameDay = date('2026-03-29T10:00:00+02:00');
+		expect(bucketByDay(justAfterDst, sameDay, 'de-DE')).toBe('today');
+	});
+});

frontend/src/lib/utils/date-buckets.ts

@@ -0,0 +1,35 @@
+export type DayBucket = 'today' | 'yesterday' | 'thisWeek' | 'older';
+
+const DAY_MS = 24 * 60 * 60 * 1000;
+const SUNDAY_START_LOCALES = new Set(['en-us', 'en-ca', 'en-ph', 'ja-jp', 'he-il', 'pt-br']);
+
+function weekStartDay(locale?: string): 0 | 1 {
+	if (!locale) return 1;
+	return SUNDAY_START_LOCALES.has(locale.toLowerCase()) ? 0 : 1;
+}
+
+function startOfDay(d: Date): Date {
+	const x = new Date(d);
+	x.setHours(0, 0, 0, 0);
+	return x;
+}
+
+function startOfWeek(d: Date, firstDay: 0 | 1): Date {
+	const x = startOfDay(d);
+	const diff = (x.getDay() - firstDay + 7) % 7;
+	x.setDate(x.getDate() - diff);
+	return x;
+}
+
+export function bucketByDay(date: Date, now: Date = new Date(), locale?: string): DayBucket {
+	const today = startOfDay(now);
+	const target = startOfDay(date);
+
+	if (target.getTime() === today.getTime()) return 'today';
+	if (today.getTime() - target.getTime() <= DAY_MS) return 'yesterday';
+
+	const weekStart = startOfWeek(today, weekStartDay(locale));
+	if (target.getTime() >= weekStart.getTime()) return 'thisWeek';
+
+	return 'older';
+}