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_de.arb

@@ -115,5 +115,9 @@
     "placeholders": {
       "count": { "type": "int" }
     }
-  }
+  },
+  "sortAlphabetical": "A\u2013Z",
+  "sortInterval": "Intervall",
+  "sortEffort": "Aufwand",
+  "sortLabel": "Sortierung"
 }