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

server/src/db/schema.ts

@@ -281,6 +281,15 @@ function createTables(db: Database.Database): void {
       UNIQUE(plan_id, date)
     );
 
+    CREATE TABLE IF NOT EXISTS vacay_holiday_calendars (
+      id        INTEGER PRIMARY KEY AUTOINCREMENT,
+      plan_id   INTEGER NOT NULL REFERENCES vacay_plans(id) ON DELETE CASCADE,
+      region    TEXT NOT NULL,
+      label     TEXT,
+      color     TEXT NOT NULL DEFAULT '#fecaca',
+      sort_order INTEGER NOT NULL DEFAULT 0
+    );
+
     CREATE TABLE IF NOT EXISTS day_accommodations (
       id INTEGER PRIMARY KEY AUTOINCREMENT,
       trip_id INTEGER NOT NULL REFERENCES trips(id) ON DELETE CASCADE,