- 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
- Add totalTaskCount field to CalendarDayState to distinguish first-run from celebration
- Add getTaskCount() to CalendarDao (SELECT COUNT from tasks)
- CalendarStrip: 181-day horizontal scroll with German abbreviations, today highlighting, month boundary labels, scroll-to-today controller
- CalendarTaskRow: task name + room tag chip + checkbox, no relative date, isOverdue coral styling
- CalendarDayList: loading/error/first-run-empty/empty-day/celebration/has-tasks states, overdue section (today only), slide-out completion animation
- Update home_screen_test.dart and app_shell_test.dart to test new calendar providers instead of dailyPlanProvider
