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.dart

@@ -519,6 +519,24 @@ abstract class AppLocalizations {
   /// In de, this message translates to:
   /// **'Heute'**
   String get calendarTodayButton;
+
+  /// No description provided for @taskHistoryTitle.
+  ///
+  /// In de, this message translates to:
+  /// **'Verlauf'**
+  String get taskHistoryTitle;
+
+  /// No description provided for @taskHistoryEmpty.
+  ///
+  /// In de, this message translates to:
+  /// **'Noch nie erledigt'**
+  String get taskHistoryEmpty;
+
+  /// No description provided for @taskHistoryCount.
+  ///
+  /// In de, this message translates to:
+  /// **'{count} Mal erledigt'**
+  String taskHistoryCount(int count);
 }
 
 class _AppLocalizationsDelegate