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/types.ts

@@ -281,10 +281,23 @@ export interface WebSocketEvent {
 }
 
 // Vacay types
+export interface VacayHolidayCalendar {
+  id: number
+  plan_id: number
+  region: string
+  label: string | null
+  color: string
+  sort_order: number
+}
+
 export interface VacayPlan {
   id: number
   holidays_enabled: boolean
   holidays_region: string | null
+  holiday_calendars: VacayHolidayCalendar[]
+  block_weekends: boolean
+  carry_over_enabled: boolean
+  company_holidays_enabled: boolean
   name?: string
   year?: number
   owner_id?: number
@@ -301,6 +314,9 @@ export interface VacayUser {
 export interface VacayEntry {
   date: string
   user_id: number
+  plan_id?: number
+  person_color?: string
+  person_name?: string
 }
 
 export interface VacayStat {
@@ -312,6 +328,8 @@ export interface VacayStat {
 export interface HolidayInfo {
   name: string
   localName: string
+  color: string
+  label: string | null
 }
 
 export interface HolidaysMap {