* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

:root {
  --bg: #f4f6fb;
  --card: #ffffff;
  --border: #d9dfeb;
  --text: #1f2937;
  --muted: #6b7280;
  --primary: #2563eb;
  --primary-dark: #1d4ed8;
  --success: #16a34a;
  --danger: #dc2626;
  --danger-dark: #b91c1c;
  --drop-bg: #eef4ff;
  --subtask-bg: #f8fafc;
  --shadow: 0 10px 30px rgba(15, 23, 42, 0.08);
}

body {
  font-family: Arial, sans-serif;
  background: var(--bg);
  color: var(--text);
  padding: 24px;
}

.app {
  max-width: 900px;
  margin: 0 auto;
  background: var(--card);
  padding: 24px;
  border-radius: 16px;
  box-shadow: var(--shadow);
}

h1 {
  text-align: center;
  margin-bottom: 24px;
  font-size: 28px;
}

.top-input-section {
  margin-bottom: 18px;
}

.top-input-row {
  display: flex;
  gap: 12px;
}

#todoInput {
  flex: 1;
  padding: 14px 16px;
  border: 1px solid var(--border);
  border-radius: 10px;
  font-size: 16px;
  outline: none;
}

#todoInput:focus {
  border-color: var(--primary);
}

#addTodoButton {
  border: none;
  background: var(--primary);
  color: white;
  padding: 14px 18px;
  border-radius: 10px;
  font-size: 15px;
  cursor: pointer;
  white-space: nowrap;
}

#addTodoButton:hover {
  background: var(--primary-dark);
}

.error-message {
  min-height: 20px;
  color: var(--danger);
  font-size: 14px;
  margin-top: 8px;
}

.filters {
  display: flex;
  gap: 10px;
  flex-wrap: wrap;
  margin-bottom: 18px;
}

.filter-btn {
  border: 1px solid var(--border);
  background: white;
  color: var(--text);
  padding: 10px 16px;
  border-radius: 999px;
  cursor: pointer;
  font-size: 14px;
}

.filter-btn.active {
  background: var(--primary);
  border-color: var(--primary);
  color: white;
}

.top-level-drop-zone {
  border: 2px dashed var(--border);
  background: #fafcff;
  color: var(--muted);
  padding: 14px;
  border-radius: 12px;
  text-align: center;
  font-size: 14px;
  margin-bottom: 18px;
  transition: 0.2s ease;
}

.top-level-drop-zone.drag-over {
  background: var(--drop-bg);
  border-color: var(--primary);
  color: var(--primary);
}

.todo-list {
  list-style: none;
  min-height: 20px;
}

.todo-item {
  list-style: none;
  border: 1px solid var(--border);
  border-radius: 14px;
  padding: 14px;
  margin-bottom: 14px;
  background: white;
  transition: 0.2s ease;
}

.todo-item.drag-over,
.subtask-item.drag-over {
  border-color: var(--primary);
  background: var(--drop-bg);
}

.todo-item.dragging,
.subtask-item.dragging {
  opacity: 0.45;
}

.todo-row {
  display: flex;
  justify-content: space-between;
  align-items: center;
  gap: 12px;
}

.todo-left {
  display: flex;
  align-items: center;
  gap: 12px;
  flex: 1;
  min-width: 0;
}

.todo-left input[type="checkbox"] {
  width: 18px;
  height: 18px;
  cursor: pointer;
  flex-shrink: 0;
}

.todo-text {
  font-size: 16px;
  word-break: break-word;
}

.todo-text.completed {
  text-decoration: line-through;
  color: var(--muted);
}

.todo-actions {
  display: flex;
  align-items: center;
  gap: 8px;
  flex-wrap: wrap;
}

.action-btn {
  border: none;
  padding: 9px 12px;
  border-radius: 8px;
  font-size: 13px;
  cursor: pointer;
}

.add-subtask-btn {
  background: var(--success);
  color: white;
}

.add-subtask-btn:hover {
  filter: brightness(0.96);
}

.delete-btn {
  background: var(--danger);
  color: white;
}

.delete-btn:hover {
  background: var(--danger-dark);
}

.subtask-input-container {
  margin-top: 12px;
  margin-left: 30px;
  padding: 12px;
  border: 1px solid var(--border);
  border-radius: 10px;
  background: #fbfcff;
}

.subtask-input-row {
  display: flex;
  gap: 10px;
}

.subtask-input {
  flex: 1;
  padding: 11px 12px;
  border: 1px solid var(--border);
  border-radius: 8px;
  font-size: 14px;
  outline: none;
}

.subtask-input:focus {
  border-color: var(--primary);
}

.subtask-add-btn {
  border: none;
  background: var(--primary);
  color: white;
  padding: 11px 14px;
  border-radius: 8px;
  cursor: pointer;
  white-space: nowrap;
}

.subtask-list {
  list-style: none;
  margin-top: 12px;
  margin-left: 30px;
}

.subtask-item {
  list-style: none;
  background: var(--subtask-bg);
  border: 1px solid #e7edf5;
  border-radius: 12px;
  padding: 12px;
  margin-bottom: 10px;
  transition: 0.2s ease;
}

.empty-state {
  text-align: center;
  color: var(--muted);
  padding: 24px 10px;
  border: 1px dashed var(--border);
  border-radius: 12px;
}

@media (max-width: 640px) {
  body {
    padding: 12px;
  }

  .app {
    padding: 16px;
  }

  .top-input-row,
  .subtask-input-row,
  .todo-row {
    flex-direction: column;
    align-items: stretch;
  }

  .todo-actions {
    justify-content: flex-start;
  }

  .subtask-input-container,
  .subtask-list {
    margin-left: 0;
  }

  #addTodoButton,
  .subtask-add-btn,
  .action-btn {
    width: 40%;
  }
}