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/features/tasks/domain/task_sort_option.dart

@@ -0,0 +1,9 @@
+/// Sort options for task lists.
+///
+/// Stored as a string in SharedPreferences (not as intEnum in the database),
+/// so reordering these values is safe.
+enum TaskSortOption {
+  alphabetical, // A–Z by task name
+  interval, // by frequency interval (most frequent first)
+  effort, // by effort level (low → medium → high)
+}

lib/features/tasks/presentation/sort_preference_notifier.dart

@@ -0,0 +1,57 @@
+import 'package:riverpod_annotation/riverpod_annotation.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+
+import 'package:household_keeper/features/tasks/domain/task_sort_option.dart';
+
+part 'sort_preference_notifier.g.dart';
+
+const _sortOptionKey = 'task_sort_option';
+
+/// Notifier that manages the active task sort preference.
+///
+/// Defaults to [TaskSortOption.alphabetical] synchronously (matching the
+/// existing A-Z sort in CalendarDayList), then loads the persisted value
+/// asynchronously on first build.
+///
+/// Follows the same pattern as [ThemeNotifier] in
+/// `lib/core/theme/theme_provider.dart`.
+@Riverpod(keepAlive: true)
+class SortPreferenceNotifier extends _$SortPreferenceNotifier {
+  @override
+  TaskSortOption build() {
+    _loadPersisted();
+    return TaskSortOption.alphabetical;
+  }
+
+  Future<void> _loadPersisted() async {
+    final prefs = await SharedPreferences.getInstance();
+    final persisted = prefs.getString(_sortOptionKey);
+    if (persisted != null) {
+      state = _fromString(persisted);
+    }
+  }
+
+  /// Update the active sort preference and persist it.
+  Future<void> setSortOption(TaskSortOption option) async {
+    state = option;
+    final prefs = await SharedPreferences.getInstance();
+    await prefs.setString(_sortOptionKey, _toString(option));
+  }
+
+  static TaskSortOption _fromString(String value) {
+    switch (value) {
+      case 'alphabetical':
+        return TaskSortOption.alphabetical;
+      case 'interval':
+        return TaskSortOption.interval;
+      case 'effort':
+        return TaskSortOption.effort;
+      default:
+        return TaskSortOption.alphabetical;
+    }
+  }
+
+  static String _toString(TaskSortOption option) {
+    return option.name;
+  }
+}

lib/features/tasks/presentation/sort_preference_notifier.g.dart

@@ -0,0 +1,96 @@
+// GENERATED CODE - DO NOT MODIFY BY HAND
+
+part of 'sort_preference_notifier.dart';
+
+// **************************************************************************
+// RiverpodGenerator
+// **************************************************************************
+
+// GENERATED CODE - DO NOT MODIFY BY HAND
+// ignore_for_file: type=lint, type=warning
+/// Notifier that manages the active task sort preference.
+///
+/// Defaults to [TaskSortOption.alphabetical] synchronously (matching the
+/// existing A-Z sort in CalendarDayList), then loads the persisted value
+/// asynchronously on first build.
+///
+/// Follows the same pattern as [ThemeNotifier] in
+/// `lib/core/theme/theme_provider.dart`.
+
+@ProviderFor(SortPreferenceNotifier)
+final sortPreferenceProvider = SortPreferenceNotifierProvider._();
+
+/// Notifier that manages the active task sort preference.
+///
+/// Defaults to [TaskSortOption.alphabetical] synchronously (matching the
+/// existing A-Z sort in CalendarDayList), then loads the persisted value
+/// asynchronously on first build.
+///
+/// Follows the same pattern as [ThemeNotifier] in
+/// `lib/core/theme/theme_provider.dart`.
+final class SortPreferenceNotifierProvider
+    extends $NotifierProvider<SortPreferenceNotifier, TaskSortOption> {
+  /// Notifier that manages the active task sort preference.
+  ///
+  /// Defaults to [TaskSortOption.alphabetical] synchronously (matching the
+  /// existing A-Z sort in CalendarDayList), then loads the persisted value
+  /// asynchronously on first build.
+  ///
+  /// Follows the same pattern as [ThemeNotifier] in
+  /// `lib/core/theme/theme_provider.dart`.
+  SortPreferenceNotifierProvider._()
+    : super(
+        from: null,
+        argument: null,
+        retry: null,
+        name: r'sortPreferenceProvider',
+        isAutoDispose: false,
+        dependencies: null,
+        $allTransitiveDependencies: null,
+      );
+
+  @override
+  String debugGetCreateSourceHash() => _$sortPreferenceNotifierHash();
+
+  @$internal
+  @override
+  SortPreferenceNotifier create() => SortPreferenceNotifier();
+
+  /// {@macro riverpod.override_with_value}
+  Override overrideWithValue(TaskSortOption value) {
+    return $ProviderOverride(
+      origin: this,
+      providerOverride: $SyncValueProvider<TaskSortOption>(value),
+    );
+  }
+}
+
+String _$sortPreferenceNotifierHash() =>
+    r'5d7f2c5d06b82b4114262ee05cf890ebe717fe2a';
+
+/// Notifier that manages the active task sort preference.
+///
+/// Defaults to [TaskSortOption.alphabetical] synchronously (matching the
+/// existing A-Z sort in CalendarDayList), then loads the persisted value
+/// asynchronously on first build.
+///
+/// Follows the same pattern as [ThemeNotifier] in
+/// `lib/core/theme/theme_provider.dart`.
+
+abstract class _$SortPreferenceNotifier extends $Notifier<TaskSortOption> {
+  TaskSortOption build();
+  @$mustCallSuper
+  @override
+  void runBuild() {
+    final ref = this.ref as $Ref<TaskSortOption, TaskSortOption>;
+    final element =
+        ref.element
+            as $ClassProviderElement<
+              AnyNotifier<TaskSortOption, TaskSortOption>,
+              TaskSortOption,
+              Object?,
+              Object?
+            >;
+    element.handleCreate(ref, build);
+  }
+}