The send flow stacked two .fileImporter modifiers on the same view (one
for files, one for folders). On iOS/macOS before 27, SwiftUI can't have
two presentation modifiers of the same kind on one view — the second
shadows the first, so toggling the files importer presented nothing and
"Choose files" appeared to do nothing. macOS/iOS 27 changed presentation
handling, which is why it worked there.
Collapse the two importers into a single .fileImporter that switches its
allowedContentTypes and allowsMultipleSelection based on whether a file
or folder pick is pending. Behavior is unchanged on 27 and now works on
26 and earlier.
Add a native SwiftUI VniDrop app (Send/Receive/Settings) talking to the
Rust core via generated UniFFI Swift bindings, plus the uniffi-bindgen
helper crate. iOS uses a TabView, macOS a NavigationSplitView sidebar.
