feat(apple): native SwiftUI app for iOS and macOS

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.

apple/VniDrop/Platform/TransferShareActions+macOS.swift

@@ -0,0 +1,48 @@
+#if os(macOS)
+import AppKit
+import SwiftUI
+
+@MainActor
+func makePlatformShareActions() -> TransferShareActions { MacTransferShareActions() }
+
+/// macOS invitation delivery, mirroring the iOS actions: save panel export and
+/// `NSSharingServicePicker` native share. NFC is unavailable on macOS.
+final class MacTransferShareActions: TransferShareActions {
+	var canUseNativeShare: Bool { true }
+	var nfcAvailability: NfcShareAvailability { .hidden }
+
+	func exportInvitation(ticket: String, transferName: String, onResult: @escaping (Result<Void, Error>) -> Void) {
+		let panel = NSSavePanel()
+		panel.nameFieldStringValue = invitationFileName(transferName)
+		panel.allowedContentTypes = []
+		panel.begin { response in
+			guard response == .OK, let url = panel.url else {
+				onResult(.failure(InvitationError.message("cancelled")))
+				return
+			}
+			onResult(Result { try ticket.write(to: url, atomically: true, encoding: .utf8) })
+		}
+	}
+
+	func shareInvitation(ticket: String, transferName: String, onResult: @escaping (Result<Void, Error>) -> Void) {
+		do {
+			let url = try writeTemporaryInvitation(ticket: ticket, transferName: transferName)
+			guard let view = NSApp.keyWindow?.contentView else {
+				onResult(.failure(InvitationError.message("No window available")))
+				return
+			}
+			let picker = NSSharingServicePicker(items: [url])
+			picker.show(relativeTo: .zero, of: view, preferredEdge: .minY)
+			onResult(.success(()))
+		} catch {
+			onResult(.failure(error))
+		}
+	}
+
+	func writeInvitationToNfc(ticket: String, onResult: @escaping (Result<Void, Error>) -> Void) {
+		onResult(.failure(InvitationError.message("NFC is unavailable on macOS")))
+	}
+
+	func cancelNfcWrite() {}
+}
+#endif