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/ReceiveInvitationActions+macOS.swift

@@ -0,0 +1,45 @@
+#if os(macOS)
+import AppKit
+import UniformTypeIdentifiers
+
+@MainActor
+func makeReceiveInvitationActions() -> ReceiveInvitationActions { MacReceiveInvitationActions() }
+
+/// macOS invitation acquisition: file picker only. QR (camera) and NFC are hidden
+/// on the desktop, matching the availability model.
+final class MacReceiveInvitationActions: ReceiveInvitationActions {
+	var fileAvailability: ReceiveMethodAvailability { .available }
+	var qrAvailability: ReceiveMethodAvailability { .hidden }
+	var nfcAvailability: ReceiveMethodAvailability { .hidden }
+
+	func pickInvitation(onResult: @escaping (Result<String, Error>) -> Void) {
+		let panel = NSOpenPanel()
+		panel.canChooseFiles = true
+		panel.canChooseDirectories = false
+		panel.allowsMultipleSelection = false
+		if let vnd = UTType(filenameExtension: vniDropInvitationExtension) {
+			panel.allowedContentTypes = [vnd, .data, .text]
+		}
+		panel.begin { response in
+			guard response == .OK, let url = panel.url else {
+				onResult(.failure(InvitationError.message("cancelled")))
+				return
+			}
+			onResult(Result {
+				let data = try Data(contentsOf: url)
+				return try decodeInvitationBytes(data)
+			})
+		}
+	}
+
+	func scanQrCode(onResult: @escaping (Result<String, Error>) -> Void) {
+		onResult(.failure(InvitationError.message("QR scanning is unavailable on macOS")))
+	}
+
+	func readNfcInvitation(onResult: @escaping (Result<String, Error>) -> Void) {
+		onResult(.failure(InvitationError.message("NFC is unavailable on macOS")))
+	}
+
+	func cancel() {}
+}
+#endif