const toggleFavorite = async (wallpaperId: string) => {
  const isFav = favorites.includes(wallpaperId)
  const newFavorites = isFav
    ? favorites.filter(id => id !== wallpaperId)
    : [...favorites, wallpaperId]

  // Optimistic update
  setFavorites(newFavorites)
  localStorage.setItem('favorites', JSON.stringify(newFavorites))

  if (isSignedIn) {
    // Sync to cloud
    await fetch('/api/favorites', {
      method: 'PATCH',
      headers: { 'Content-Type': 'application/json' },
      body: JSON.stringify({
        wallpaperId,
        action: isFav ? 'remove' : 'add'
      })
    })
  }
}

const syncLocalToCloud = async () => {
  if (!isSignedIn) return

  const localFavorites = JSON.parse(localStorage.getItem('favorites'))
  const response = await fetch('/api/favorites')
  const data = await response.json()
  const cloudFavorites = data.favorites || []
  
  // Merge without duplicates
  const merged = [...new Set([...cloudFavorites, ...localFavorites])]
  
  // Save merged favorites to cloud
  await fetch('/api/favorites', {
    method: 'POST',
    body: JSON.stringify({ favorites: merged })
  })
  
  setFavorites(merged)
  localStorage.setItem('favorites', JSON.stringify(merged))
}