fix: 할일 마감일 타임존 정규화

- 할일 생성/수정 시 마감일을 KST 기준 정오(12:00)로 정규화
- 클라이언트에서 전송된 날짜가 날짜 경계를 벗어나는 문제 해결

backend/routes/todos.js

@@ -63,9 +63,22 @@ router.get("/today", async (req, res) => {
   }
 });
 
+// Normalize dueDate to noon KST to avoid timezone boundary issues
+const normalizeDueDate = (dueDate) => {
+  if (!dueDate) return null;
+  // Parse the date and set it to noon KST
+  const date = DateTime.fromISO(dueDate, { zone: KST_TIMEZONE });
+  // Set to noon (12:00) of that day in KST
+  return date.set({ hour: 12, minute: 0, second: 0, millisecond: 0 }).toJSDate();
+};
+
 router.post("/", async (req, res) => {
   try {
-    const todo = await Todo.create(req.body);
+    const todoData = { ...req.body };
+    if (todoData.dueDate) {
+      todoData.dueDate = normalizeDueDate(todoData.dueDate);
+    }
+    const todo = await Todo.create(todoData);
     res.status(201).json(todo);
   } catch (error) {
     res.status(400).json({ message: "Failed to create todo" });
@@ -86,7 +99,11 @@ router.get("/:id", async (req, res) => {
 
 router.put("/:id", async (req, res) => {
   try {
-    const todo = await Todo.findByIdAndUpdate(req.params.id, req.body, { new: true });
+    const updateData = { ...req.body };
+    if (updateData.dueDate) {
+      updateData.dueDate = normalizeDueDate(updateData.dueDate);
+    }
+    const todo = await Todo.findByIdAndUpdate(req.params.id, updateData, { new: true });
     if (!todo) {
       return res.status(404).json({ message: "Todo not found" });
     }