More ergonomic SwiftUI API when a view has many things to toast

For example, if a view has several error states, it'd be more ergonomic to have one error object which can be set, and if so, a toast appears.

```swift
HStack {
// ...
}
.toast(item: $currentError) { currentError in
Text(currentError.localizedDescription)
}
```

The issue provides a SwiftUI call site using `.toast(item: $currentError)` and an error value's `localizedDescription`, but names no files or tests. Start by locating the existing toast API and its tests, then determine how an optional error item should drive presentation. Done means the shown usage works for multiple error states without separate toast calls.

Written by the indexing model from the issue text.