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

:root {
  --primary-color: #2196F3;
  --primary-dark: #1976D2;
  --success-color: #4CAF50;
  --danger-color: #f44336;
  --text-primary: #212121;
  --text-secondary: #757575;
  --bg-primary: #ffffff;
  --bg-secondary: #f5f5f5;
  --border-color: #e0e0e0;
  --shadow: 0 2px 4px rgba(0,0,0,0.1);
  --shadow-lg: 0 4px 8px rgba(0,0,0,0.15);
}

body {
  font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, sans-serif;
  background: var(--bg-secondary);
  color: var(--text-primary);
  line-height: 1.6;
  min-height: 100vh;
}

.screen {
  display: block;
}

.screen.hidden {
  display: none;
}

/* Login Screen */
.login-container {
  max-width: 400px;
  margin: 100px auto;
  padding: 40px;
  background: var(--bg-primary);
  border-radius: 8px;
  box-shadow: var(--shadow-lg);
  text-align: center;
}

.login-container h1 {
  margin-bottom: 30px;
  color: var(--text-primary);
}

#loginForm {
  display: flex;
  flex-direction: column;
  gap: 15px;
}

#loginForm input {
  padding: 12px;
  border: 2px solid var(--border-color);
  border-radius: 4px;
  font-size: 16px;
  transition: border-color 0.3s;
}

#loginForm input:focus {
  outline: none;
  border-color: var(--primary-color);
}

#loginForm button {
  padding: 12px;
  background: var(--primary-color);
  color: white;
  border: none;
  border-radius: 4px;
  font-size: 16px;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.3s;
}

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

.error {
  color: var(--danger-color);
  margin-top: 10px;
  font-size: 14px;
}

/* Main App */
.container {
  max-width: 600px;
  margin: 0 auto;
  padding: 20px;
}

header {
  display: flex;
  justify-content: space-between;
  align-items: center;
  margin-bottom: 30px;
  padding-bottom: 20px;
  border-bottom: 2px solid var(--primary-color);
}

header h1 {
  font-size: 28px;
  color: var(--primary-color);
}

.header-actions {
  display: flex;
  gap: 10px;
}

.icon-btn {
  background: none;
  border: none;
  font-size: 24px;
  cursor: pointer;
  padding: 5px;
  transition: transform 0.2s;
}

.icon-btn:hover {
  transform: scale(1.1);
}

.icon-btn:active {
  transform: scale(0.95);
}

#syncIcon.spinning {
  display: inline-block;
  animation: spin 1s linear infinite;
}

@keyframes spin {
  from { transform: rotate(0deg); }
  to { transform: rotate(360deg); }
}

/* Add Todo Form */
.add-todo {
  margin-bottom: 20px;
}

#addTodoForm {
  display: flex;
  gap: 10px;
}

#todoInput {
  flex: 1;
  padding: 12px;
  border: 2px solid var(--border-color);
  border-radius: 4px;
  font-size: 16px;
  transition: border-color 0.3s;
}

#todoInput:focus {
  outline: none;
  border-color: var(--primary-color);
}

#addTodoForm button {
  padding: 12px 24px;
  background: var(--primary-color);
  color: white;
  border: none;
  border-radius: 4px;
  font-size: 16px;
  font-weight: 600;
  cursor: pointer;
  transition: background 0.3s;
}

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

/* Stats */
.stats {
  display: flex;
  justify-content: space-around;
  padding: 15px;
  background: var(--bg-primary);
  border-radius: 4px;
  margin-bottom: 15px;
  box-shadow: var(--shadow);
}

.stats span {
  font-size: 14px;
  color: var(--text-secondary);
  font-weight: 500;
}

/* Filter Tabs */
.filter-tabs {
  display: flex;
  gap: 10px;
  margin-bottom: 20px;
}

.filter-tab {
  flex: 1;
  padding: 10px;
  background: var(--bg-primary);
  border: 2px solid var(--border-color);
  border-radius: 4px;
  cursor: pointer;
  font-size: 14px;
  font-weight: 500;
  transition: all 0.3s;
}

.filter-tab:hover {
  border-color: var(--primary-color);
}

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

/* Todo List */
.todo-list {
  list-style: none;
}

.todo-item {
  display: flex;
  align-items: center;
  gap: 12px;
  padding: 15px;
  background: var(--bg-primary);
  border-radius: 4px;
  margin-bottom: 10px;
  box-shadow: var(--shadow);
  transition: all 0.3s;
}

.todo-item:hover {
  box-shadow: var(--shadow-lg);
}

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

.todo-checkbox {
  width: 24px;
  height: 24px;
  cursor: pointer;
  flex-shrink: 0;
}

.todo-text {
  flex: 1;
  font-size: 16px;
  word-break: break-word;
}

.delete-btn {
  background: var(--danger-color);
  color: white;
  border: none;
  border-radius: 4px;
  padding: 6px 12px;
  font-size: 14px;
  cursor: pointer;
  transition: all 0.3s;
  flex-shrink: 0;
}

.delete-btn:hover {
  background: #d32f2f;
}

.delete-btn:active {
  transform: scale(0.95);
}

/* Sync Status */
.sync-status {
  text-align: center;
  margin-top: 30px;
  padding: 15px;
  background: var(--bg-primary);
  border-radius: 4px;
  font-size: 14px;
  color: var(--text-secondary);
}

#syncStatus {
  font-weight: 600;
  color: var(--success-color);
}

#syncStatus.syncing {
  color: var(--primary-color);
}

#syncStatus.error {
  color: var(--danger-color);
}

/* Responsive */
@media (max-width: 640px) {
  .container {
    padding: 15px;
  }

  header h1 {
    font-size: 24px;
  }

  .stats {
    flex-direction: column;
    gap: 8px;
    text-align: center;
  }

  #addTodoForm {
    flex-direction: column;
  }

  #addTodoForm button {
    width: 100%;
  }
}

/* Empty State */
.empty-state {
  text-align: center;
  padding: 60px 20px;
  color: var(--text-secondary);
}

.empty-state-icon {
  font-size: 64px;
  margin-bottom: 20px;
}

.empty-state-text {
  font-size: 18px;
}