feat(05-01): add CalendarDayState model, Riverpod providers, and l10n strings

- CalendarDayState: selectedDate, dayTasks, overdueTasks fields with isEmpty helper
- selectedDateProvider: NotifierProvider with SelectedDateNotifier, defaults to today
- calendarDayProvider: StreamProvider.autoDispose, overdue only when viewing today
- Add calendarTodayButton l10n string ("Heute") to ARB and generated dart files

lib/features/home/domain/calendar_models.dart

@@ -0,0 +1,19 @@
+import 'package:household_keeper/features/home/domain/daily_plan_models.dart';
+
+/// State for the calendar day view: tasks for the selected date + overdue tasks.
+class CalendarDayState {
+  final DateTime selectedDate;
+  final List<TaskWithRoom> dayTasks;
+  final List<TaskWithRoom> overdueTasks;
+
+  const CalendarDayState({
+    required this.selectedDate,
+    required this.dayTasks,
+    required this.overdueTasks,
+  });
+
+  /// True when both day tasks and overdue tasks are empty.
+  /// Determined by the UI layer (completion state vs. no tasks at all
+  /// is handled in the widget based on this flag and history context).
+  bool get isEmpty => dayTasks.isEmpty && overdueTasks.isEmpty;
+}