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/features/tasks/data/tasks_dao.dart

@@ -81,6 +81,14 @@ class TasksDao extends DatabaseAccessor<AppDatabase> with _$TasksDaoMixin {
     });
   }
 
+  /// Watch all completions for a task, newest first.
+  Stream<List<TaskCompletion>> watchCompletionsForTask(int taskId) {
+    return (select(taskCompletions)
+          ..where((c) => c.taskId.equals(taskId))
+          ..orderBy([(c) => OrderingTerm.desc(c.completedAt)]))
+        .watch();
+  }
+
   /// Count overdue tasks in a room (nextDueDate before today).
   Future<int> getOverdueTaskCount(int roomId, {DateTime? today}) async {
     final now = today ?? DateTime.now();