feat(06-01): add watchCompletionsForTask DAO method and history localization strings

- Add watchCompletionsForTask(taskId) to TasksDao: Stream<List<TaskCompletion>> sorted newest first
- Regenerate tasks_dao.g.dart with build_runner
- Add taskHistoryTitle, taskHistoryEmpty, taskHistoryCount to app_de.arb
- Regenerate app_localizations.dart and app_localizations_de.dart
- All 5 new DAO tests pass, zero analyze issues

lib/l10n/app_localizations_de.dart

@@ -239,4 +239,15 @@ class AppLocalizationsDe extends AppLocalizations {
 
   @override
   String get calendarTodayButton => 'Heute';
+
+  @override
+  String get taskHistoryTitle => 'Verlauf';
+
+  @override
+  String get taskHistoryEmpty => 'Noch nie erledigt';
+
+  @override
+  String taskHistoryCount(int count) {
+    return '$count Mal erledigt';
+  }
 }