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/l10n/app_localizations_de.dart

@@ -236,4 +236,7 @@ class AppLocalizationsDe extends AppLocalizations {
   String notificationBodyWithOverdue(int count, int overdue) {
     return '$count Aufgaben fällig ($overdue überfällig)';
   }
+
+  @override
+  String get calendarTodayButton => 'Heute';
 }