Create email_design content from HTML or ZIP file and return the UUID of the created content.

ZIP File Structure: When uploading a ZIP file, it must follow this structure:

design.zip
├── index.html or mail.html         (required - main HTML file)
└── images/             (optional - image assets folder)
    ├── logo.png
    ├── banner.jpg
    └── icon.gif

Supported image formats: PNG, JPG, JPEG, AVIF, GIF, JFIF, WEBP, WOFF

Requirements:

• ZIP file must contain index.html at the root level
• All images should be placed in the images/ folder or directly on the root level ./
• Image references in HTML should use relative paths: images/filename.ext