feat: multiple holiday calendars per vacay plan

- Add vacay_holiday_calendars table (region, label, color, sort_order)
- Lazy migration of existing holidays_region to first calendar row
- Extract applyHolidayCalendars() helper; replace inline holiday logic
- GET /vacay/plan now includes holiday_calendars array
- Add POST/PUT/DELETE /vacay/plan/holiday-calendars/:id endpoints
- Client VacayPlan/VacayEntry/HolidayInfo types updated
- loadHolidays() loops over all calendars; per-calendar color on HolidayInfo
- VacayMonthCard uses holiday.color instead of hardcoded red
- VacaySettings replaced single country picker with calendar list UI
- VacayPage legend renders one item per calendar
- i18n: addCalendar, calendarLabel, calendarColor, noCalendars (en + de)
- Fix pre-existing TS errors: VacayPlan/VacayEntry missing fields,
  SettingToggleProps icon/onChange types, packing.suggestions.items array type

Closes #36

client/src/components/Vacay/VacayMonthCard.tsx

@@ -8,6 +8,13 @@ const WEEKDAYS_DE = ['Mo', 'Di', 'Mi', 'Do', 'Fr', 'Sa', 'So']
 const MONTHS_EN = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December']
 const MONTHS_DE = ['Januar', 'Februar', 'März', 'April', 'Mai', 'Juni', 'Juli', 'August', 'September', 'Oktober', 'November', 'Dezember']
 
+function hexToRgba(hex: string, alpha: number): string {
+  const r = parseInt(hex.slice(1, 3), 16)
+  const g = parseInt(hex.slice(3, 5), 16)
+  const b = parseInt(hex.slice(5, 7), 16)
+  return `rgba(${r},${g},${b},${alpha})`
+}
+
 interface VacayMonthCardProps {
   year: number
   month: number
@@ -86,7 +93,7 @@ export default function VacayMonthCard({
                   onMouseEnter={e => { if (!isBlocked) e.currentTarget.style.background = 'var(--bg-hover)' }}
                   onMouseLeave={e => { e.currentTarget.style.background = weekend ? 'var(--bg-secondary)' : 'transparent' }}
                 >
-                  {holiday && <div className="absolute inset-0.5 rounded" style={{ background: 'rgba(239,68,68,0.12)' }} />}
+                  {holiday && <div className="absolute inset-0.5 rounded" style={{ background: hexToRgba(holiday.color, 0.12) }} />}
                   {isCompany && <div className="absolute inset-0.5 rounded" style={{ background: 'rgba(245,158,11,0.15)' }} />}
 
                   {dayEntries.length === 1 && (
@@ -115,7 +122,7 @@ export default function VacayMonthCard({
                   )}
 
                   <span className="relative z-[1] text-[11px] font-medium" style={{
-                    color: holiday ? '#dc2626' : weekend ? 'var(--text-faint)' : 'var(--text-primary)',
+                    color: holiday ? holiday.color : weekend ? 'var(--text-faint)' : 'var(--text-primary)',
                     fontWeight: dayEntries.length > 0 ? 700 : 500,
                   }}>
                     {day}