fix: 마감일 없는 할일도 완료 여부와 상관없이 표시

- 마감일이 없는 할일은 특정 날짜에 속하지 않으므로 항상 표시
- 완료 체크 시 목록에서 사라지는 버그 수정

backend/routes/todos.js

@@ -40,7 +40,7 @@ router.get("/today", async (req, res) => {
     // Show todos that are:
     // 1. Due today (between start and end of today) - both completed and incomplete
     // 2. Due in the future (not yet due) - only incomplete
-    // 3. Have no due date set - only incomplete
+    // 3. Have no due date set - both completed and incomplete (always show)
     // Filter out: overdue todos (due date before today)
     const todos = await Todo.find({
       $or: [
@@ -48,9 +48,9 @@ router.get("/today", async (req, res) => {
         { dueDate: { $gte: start, $lte: end } },
         // Future todos - only incomplete
         { dueDate: { $gt: end }, completed: false },
-        // No due date - only incomplete
-        { dueDate: { $exists: false }, completed: false },
-        { dueDate: null, completed: false }
+        // No due date - always show (completed or not)
+        { dueDate: { $exists: false } },
+        { dueDate: null }
       ]
     }).sort({
       completed: 1,  // Incomplete first

flutter_app/lib/services/todo_service.dart

@@ -39,8 +39,8 @@ class TodoService {
           // Future date - only show if incomplete
           return !todo.completed;
         }
-        // No due date - only show if incomplete
-        return !todo.completed;
+        // No due date - always show (completed or not)
+        return true;
       }).toList();
 
       // Sort: incomplete first, then by due date