feat(05-02): build CalendarStrip, CalendarTaskRow, CalendarDayList widgets

- 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

lib/features/home/domain/calendar_models.dart

@@ -6,10 +6,16 @@ class CalendarDayState {
   final List<TaskWithRoom> dayTasks;
   final List<TaskWithRoom> overdueTasks;
 
+  /// Total number of tasks in the database (across all days/rooms).
+  /// Used by the UI to distinguish first-run empty state (no tasks exist at all)
+  /// from celebration state (tasks exist but today's are all done).
+  final int totalTaskCount;
+
   const CalendarDayState({
     required this.selectedDate,
     required this.dayTasks,
     required this.overdueTasks,
+    required this.totalTaskCount,
   });
 
   /// True when both day tasks and overdue tasks are empty.