大文件分片上传

async function uploadLargeFile(file, chunkSize = 1024 * 1024) {
  const totalChunks = Math.ceil(file.size / chunkSize);
  const fileId = Date.now().toString();
  
  for (let chunkIndex = 0; chunkIndex < totalChunks; chunkIndex++) {
    const start = chunkIndex * chunkSize;
    const end = Math.min(start + chunkSize, file.size);
    const chunk = file.slice(start, end);
    
    const formData = new FormData();
    formData.append('file', chunk);
    formData.append('chunkIndex', chunkIndex);
    formData.append('totalChunks', totalChunks);
    formData.append('fileId', fileId);
    formData.append('fileName', file.name);
    
    await fetch('/upload-chunk', {
      method: 'POST',
      body: formData
    });
    
    updateProgress((chunkIndex + 1) / totalChunks * 100);
  }
  
  // 通知服务器合并文件
  await fetch('/merge-file', {
    method: 'POST',
    headers: { 'Content-Type': 'application/json' },
    body: JSON.stringify({ fileId, fileName: file.name })
  });
}