Phase 0.3: migrate date formatting to date-fns

frontend/src/pages/Calendar.tsx

@@ -18,6 +18,7 @@ import { MonthView } from '@/components/calendar/MonthView';
 import { WeekView } from '@/components/calendar/WeekView';
 import { DayView } from '@/components/calendar/DayView';
 import { RangeView } from '@/components/calendar/RangeView';
+import { formatDateInput } from '@/utils/date';
 import { CalendarDetail } from '@/components/calendar/CalendarDetail';
 import { AppointmentModal } from '@/components/calendar/AppointmentModal';
 import { IcsControls } from '@/components/calendar/IcsControls';
@@ -488,7 +489,7 @@ export function CalendarPage() {
         <label className="text-xs text-secondary-600">{t('calendar.range.start')}</label>
         <input
           type="date"
-          value={rangeStart ? rangeStart.toISOString().slice(0, 10) : ''}
+          value={rangeStart ? formatDateInput(rangeStart) : ''}
           onChange={(e) => setRangeStart(e.target.value ? new Date(e.target.value) : null)}
           className="text-sm rounded-md border border-secondary-300 px-2 py-1"
           data-testid="calendar-range-start"
@@ -496,7 +497,7 @@ export function CalendarPage() {
         <label className="text-xs text-secondary-600">{t('calendar.range.end')}</label>
         <input
           type="date"
-          value={rangeEnd ? rangeEnd.toISOString().slice(0, 10) : ''}
+          value={rangeEnd ? formatDateInput(rangeEnd) : ''}
           onChange={(e) => setRangeEnd(e.target.value ? new Date(e.target.value) : null)}
           className="text-sm rounded-md border border-secondary-300 px-2 py-1"
           data-testid="calendar-range-end"