Add Favorites Feature for Artwork & Tours

Users currently cannot bookmark or favorite artworks or tours for later reference. Adding a “favorites” feature will improve personalization and allow quick access to preferred items.
Sample:
```swift
// Example: toggling favorite status
struct FavoriteItem: Codable { let id: Int; let type: String }
UserDefaults.standard.toggleFavorite(FavoriteItem(id: 1036, type: "object"))
```