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/presentation/calendar_providers.dart

@@ -0,0 +1,66 @@
+import 'package:flutter_riverpod/flutter_riverpod.dart';
+
+import 'package:household_keeper/core/providers/database_provider.dart';
+import 'package:household_keeper/features/home/domain/calendar_models.dart';
+import 'package:household_keeper/features/home/domain/daily_plan_models.dart';
+
+/// Notifier that manages the currently selected date in the calendar strip.
+///
+/// Defaults to today (start of day, time zeroed out).
+/// NOT autoDispose — the selected date persists while the app is alive.
+class SelectedDateNotifier extends Notifier<DateTime> {
+  @override
+  DateTime build() {
+    final now = DateTime.now();
+    return DateTime(now.year, now.month, now.day);
+  }
+
+  /// Update the selected date (always normalized to start of day).
+  void selectDate(DateTime date) {
+    state = DateTime(date.year, date.month, date.day);
+  }
+}
+
+/// Provider for the currently selected date in the calendar strip.
+final selectedDateProvider =
+    NotifierProvider<SelectedDateNotifier, DateTime>(
+  SelectedDateNotifier.new,
+);
+
+/// Reactive calendar day state: tasks for the selected date + overdue tasks.
+///
+/// Overdue tasks are only included when the selected date is today.
+/// Past and future dates show only tasks originally due on that day.
+///
+/// Defined manually (not @riverpod) because riverpod_generator has trouble
+/// with drift's generated [Task] type. Same pattern as [dailyPlanProvider].
+final calendarDayProvider =
+    StreamProvider.autoDispose<CalendarDayState>((ref) {
+  final db = ref.watch(appDatabaseProvider);
+  final selectedDate = ref.watch(selectedDateProvider);
+
+  final now = DateTime.now();
+  final today = DateTime(now.year, now.month, now.day);
+  final isToday = selectedDate == today;
+
+  final dayTasksStream = db.calendarDao.watchTasksForDate(selectedDate);
+
+  return dayTasksStream.asyncMap((dayTasks) async {
+    final List<TaskWithRoom> overdueTasks;
+
+    if (isToday) {
+      // When viewing today, include overdue tasks (due before today)
+      overdueTasks =
+          await db.calendarDao.watchOverdueTasks(selectedDate).first;
+    } else {
+      // Past or future dates: no overdue carry-over
+      overdueTasks = const [];
+    }
+
+    return CalendarDayState(
+      selectedDate: selectedDate,
+      dayTasks: dayTasks,
+      overdueTasks: overdueTasks,
+    );
+  });
+});