feat(07-01): create TaskSortOption enum, SortPreferenceNotifier, and localization strings

- TaskSortOption enum with alphabetical, interval, effort values
- SortPreferenceNotifier persists sort preference to SharedPreferences
- Follows ThemeNotifier pattern: sync default (alphabetical), async load
- Generated sort_preference_notifier.g.dart via build_runner
- Added sortAlphabetical/sortInterval/sortEffort/sortLabel to app_de.arb
- Regenerated app_localizations.dart and app_localizations_de.dart

lib/l10n/app_localizations_de.dart

@@ -250,4 +250,16 @@ class AppLocalizationsDe extends AppLocalizations {
   String taskHistoryCount(int count) {
     return '$count Mal erledigt';
   }
+
+  @override
+  String get sortAlphabetical => 'A–Z';
+
+  @override
+  String get sortInterval => 'Intervall';
+
+  @override
+  String get sortEffort => 'Aufwand';
+
+  @override
+  String get sortLabel => 'Sortierung';
 }