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.dart

@@ -537,6 +537,30 @@ abstract class AppLocalizations {
   /// In de, this message translates to:
   /// **'{count} Mal erledigt'**
   String taskHistoryCount(int count);
+
+  /// No description provided for @sortAlphabetical.
+  ///
+  /// In de, this message translates to:
+  /// **'A–Z'**
+  String get sortAlphabetical;
+
+  /// No description provided for @sortInterval.
+  ///
+  /// In de, this message translates to:
+  /// **'Intervall'**
+  String get sortInterval;
+
+  /// No description provided for @sortEffort.
+  ///
+  /// In de, this message translates to:
+  /// **'Aufwand'**
+  String get sortEffort;
+
+  /// No description provided for @sortLabel.
+  ///
+  /// In de, this message translates to:
+  /// **'Sortierung'**
+  String get sortLabel;
 }
 
 class _AppLocalizationsDelegate