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

flutter_app/lib/services/todo_service.dart

@@ -22,23 +22,39 @@ class TodoService {
     if (ApiConfig.useMockData) {
       // Using device's local timezone (should be Asia/Seoul for Korean users)
       final now = DateTime.now();
-      final today = DateTime(now.year, now.month, now.day);
-      final tomorrow = today.add(const Duration(days: 1));
+      final startOfToday = DateTime(now.year, now.month, now.day);
+      final endOfToday = DateTime(now.year, now.month, now.day, 23, 59, 59, 999);
 
-      return MockDataStore.todos.where((todo) {
-        if (todo.completed) {
-          // If completed, only show if it was due today
-          if (todo.dueDate == null) return false;
-          return todo.dueDate!.isAfter(today.subtract(const Duration(milliseconds: 1))) &&
-                 todo.dueDate!.isBefore(tomorrow);
+      final filtered = MockDataStore.todos.where((todo) {
+        if (todo.dueDate != null) {
+          // Due today - show regardless of completion status
+          if (todo.dueDate!.isAfter(startOfToday.subtract(const Duration(milliseconds: 1))) &&
+              todo.dueDate!.isBefore(endOfToday.add(const Duration(milliseconds: 1)))) {
+            return true;
+          }
+          // Overdue (before today) - filter out
+          if (todo.dueDate!.isBefore(startOfToday)) {
+            return false;
+          }
+          // Future date - only show if incomplete
+          return !todo.completed;
         }
-        
-        // If not completed:
-        // 1. No due date -> show
-        // 2. Due today or before -> show
-        if (todo.dueDate == null) return true;
-        return todo.dueDate!.isBefore(tomorrow);
+        // No due date - only show if incomplete
+        return !todo.completed;
       }).toList();
+
+      // Sort: incomplete first, then by due date
+      filtered.sort((a, b) {
+        if (a.completed != b.completed) {
+          return a.completed ? 1 : -1;
+        }
+        if (a.dueDate == null && b.dueDate == null) return 0;
+        if (a.dueDate == null) return 1;
+        if (b.dueDate == null) return -1;
+        return a.dueDate!.compareTo(b.dueDate!);
+      });
+
+      return filtered;
     }
     final data = await _client.getList("${ApiConfig.todos}/today");
     return data