Web Storage (localStorage, sessionStorage, cookies)

const syncStorage = {
  async set(key, value) {
    localStorage.setItem(key, JSON.stringify(value));
    try {
      await fetch('/api/sync', {
        method: 'POST',
        body: JSON.stringify({ key, value }),
        headers: { 'Content-Type': 'application/json' }
      });
    } catch (e) {
      // Mark for sync later
      this.addToSyncQueue(key);
    }
  },
  
  addToSyncQueue(key) {
    const queue = JSON.parse(localStorage.getItem('_syncQueue') || '[]');
    queue.push(key);
    localStorage.setItem('_syncQueue', JSON.stringify(queue));
  }
};