- Add watchAllActiveRecurringTasks() and watchAllActiveRecurringTasksInRoom() to CalendarDao
- Add watchCompletionsInRange() for period-completion filtering
- Extend CalendarDayState with prePopulatedTasks field (default empty, backward compat)
- Update isEmpty getter to include prePopulatedTasks.isEmpty
- Add _isInCurrentIntervalWindow() and _calculatePreviousDueDate() helpers
- Rewrite calendarDayProvider and roomCalendarDayProvider with pre-population logic
- Fix _subtractMonths() year-boundary bug using total-month arithmetic
- Add 9 new DAO tests for watchAllActiveRecurringTasks, watchAllActiveRecurringTasksInRoom, watchCompletionsInRange