refactor(todo): 移除 due_at 字段，改用 order 字段管理象限内顺序

apps/lib/features/todo/ui/widgets/todo_drag_item.dart

@@ -5,6 +5,7 @@ import 'package:social_app/features/todo/data/todo_api.dart';
 class TodoDragItem extends StatelessWidget {
   final TodoResponse todo;
   final int quadrant;
+  final int sourceIndex;
   final VoidCallback onDragStarted;
   final VoidCallback onDragEnd;
   final Widget child;
@@ -13,6 +14,7 @@ class TodoDragItem extends StatelessWidget {
     super.key,
     required this.todo,
     required this.quadrant,
+    required this.sourceIndex,
     required this.onDragStarted,
     required this.onDragEnd,
     required this.child,
@@ -21,7 +23,7 @@ class TodoDragItem extends StatelessWidget {
   @override
   Widget build(BuildContext context) {
     return LongPressDraggable<String>(
-      data: '${todo.id}:$quadrant',
+      data: '${todo.id}:$quadrant:$sourceIndex',
       delay: const Duration(milliseconds: 150),
       feedback: Material(
         elevation: 8,