Migrate to SwiftUI and Swift Concurrency

Migrate app architecture to SwiftUI and modern Swift concurrency.

Migration Tasks:

• Replace UIKit views with SwiftUI
• Convert callbacks to async/await
• Replace completion handlers with async functions
• Implement proper error handling
• Update networking layer
• Add Combine publishers where appropriate
• Update tests for async code

Benefits:

• Modern, declarative UI
• Better state management
• Cleaner async code
• Improved performance

Timeline: Phased migration over 2 sprints