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_de.arb

@@ -106,5 +106,6 @@
       "count": { "type": "int" },
       "overdue": { "type": "int" }
     }
-  }
+  },
+  "calendarTodayButton": "Heute"
 }

lib/l10n/app_localizations.dart

@@ -513,6 +513,12 @@ abstract class AppLocalizations {
   /// In de, this message translates to:
   /// **'{count} Aufgaben fällig ({overdue} überfällig)'**
   String notificationBodyWithOverdue(int count, int overdue);
+
+  /// No description provided for @calendarTodayButton.
+  ///
+  /// In de, this message translates to:
+  /// **'Heute'**
+  String get calendarTodayButton;
 }
 
 class _AppLocalizationsDelegate

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';
 }