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/UI/Navigation/AppDestination.swift

@@ -0,0 +1,27 @@
+import Foundation
+
+/// Top-level destinations, ported from `ui/navigation/AppDestination.kt`.
+enum AppDestination: String, CaseIterable, Identifiable {
+	case send
+	case receive
+	case settings
+
+	var id: String { rawValue }
+
+	var labelKey: String {
+		switch self {
+		case .send: return "nav_send"
+		case .receive: return "nav_receive"
+		case .settings: return "nav_settings"
+		}
+	}
+
+	/// SF Symbol approximating the Compose line icon.
+	var systemImage: String {
+		switch self {
+		case .send: return "paperplane"
+		case .receive: return "tray.and.arrow.down"
+		case .settings: return "gearshape"
+		}
+	}
+}