feat: add appointment calendar and cross-customer booking

src/modules/appointment-calendar/calendar.ts

@@ -0,0 +1,45 @@
+export type CalendarMode = 'month' | 'week'
+
+/** Calendar dates are warehouse-local dates, never UTC conversions. */
+export function dateKey(date: Date): string {
+  return `${date.getFullYear()}-${String(date.getMonth() + 1).padStart(2, '0')}-${String(date.getDate()).padStart(2, '0')}`
+}
+
+function parseDate(value: string): Date {
+  const date = new Date(`${value}T12:00:00`)
+  if (!/^\d{4}-\d{2}-\d{2}$/.test(value) || Number.isNaN(date.getTime()) || dateKey(date) !== value) {
+    throw new Error('无效的日历日期')
+  }
+  return date
+}
+
+export function addDays(value: string, days: number): string {
+  const date = parseDate(value)
+  date.setDate(date.getDate() + days)
+  return dateKey(date)
+}
+
+export function calendarRange(anchor: string, mode: CalendarMode): { from: string; to: string } {
+  const date = parseDate(anchor)
+  if (mode === 'month') date.setDate(1)
+  date.setDate(date.getDate() - (date.getDay() + 6) % 7)
+  const from = dateKey(date)
+  // A half-open interval also works directly with database timestamps.
+  return { from, to: addDays(from, mode === 'month' ? 42 : 7) }
+}
+
+export function calendarDates(anchor: string, mode: CalendarMode): string[] {
+  const { from } = calendarRange(anchor, mode)
+  return Array.from({ length: mode === 'month' ? 42 : 7 }, (_, index) => addDays(from, index))
+}
+
+export function moveCalendar(anchor: string, mode: CalendarMode, offset: number): string {
+  if (mode === 'week') return addDays(anchor, offset * 7)
+  const date = parseDate(anchor)
+  const day = date.getDate()
+  date.setDate(1)
+  date.setMonth(date.getMonth() + offset)
+  const lastDay = new Date(date.getFullYear(), date.getMonth() + 1, 0).getDate()
+  date.setDate(Math.min(day, lastDay))
+  return dateKey(date)
+}