feat: 오늘의 할일 완료 항목 표시 개선

- 오늘 마감 할일은 완료해도 취소선으로 표시하며 유지
- 날짜가 지난 할일만 필터링하여 숨김
- 미완료 항목을 먼저 표시하도록 정렬 순서 변경
- 백엔드와 Flutter Mock 데이터 로직 동기화

backend/routes/todos.js

@@ -36,18 +36,24 @@ router.get("/", async (req, res) => {
 
 router.get("/today", async (req, res) => {
   try {
-    const { start } = getDayRange();
-    // Show incomplete todos that are:
-    // 1. Due today or later (not overdue)
-    // 2. Have no due date set
+    const { start, end } = getDayRange();
+    // 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
+    // Filter out: overdue todos (due date before today)
     const todos = await Todo.find({
-      completed: false,
       $or: [
-        { dueDate: { $gte: start } },
-        { dueDate: { $exists: false } },
-        { dueDate: null }
+        // Today's todos (completed or not) - show with strikethrough if completed
+        { 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 }
       ]
     }).sort({
+      completed: 1,  // Incomplete first
       dueDate: 1,
       createdAt: -1,
     });