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)
+}