Create a complete SwiftUI view for a user profile card.

The view should be named `UserProfileCard` and be initialized with a simple `User` struct containing `name: String`, `handle: String`, and `avatarURL: URL`.

Requirements:
- Use an `AsyncImage` to load the user's avatar from the URL.
- Display the user's name in a `.headline` font and their handle (e.g., "@username") in a `.subheadline` font with a secondary color.
- Include a "Follow" button that uses the "person.badge.plus" SF Symbol.
- Arrange the elements cleanly inside an HStack with appropriate spacing.
- The entire card should have a subtle shadow and rounded corners.