I've created a modern video downloader interface with:

1. A clean, gradient background with a centered layout
2. A URL input field with a download button
3. Quality and format selection dropdowns
4. Supported platforms displayed as cards with icons
5. Step-by-step instructions
6. A basic API route structure for handling downloads

Note that this is just the frontend interface - implementing actual video downloading functionality would require additional backend services and proper handling of video processing. The API route is included as a starting point for integrating with a video processing service.

You can customize the styling, add more features like download progress indicators, or integrate with specific video platform APIs based on your needs.

This Flutter implementation includes:

1. A responsive layout that works on both mobile and tablet devices
2. Dark theme with gradient background
3. Custom styled input fields for URL entry
4. Dropdown selectors for quality and format options
5. Platform cards showing supported services
6. Basic dependencies setup in pubspec.yaml

To complete the implementation, you would need to:

1. Add state management (e.g., Provider or Bloc) for handling downloads
2. Implement the actual download functionality
3. Add progress indicators and error handling
4. Implement platform-specific storage permissions
5. Add download history and management features

The UI is fully responsive and will adapt to different screen sizes. The app uses Material Design components with custom styling to match the Snaptube aesthetic.