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/Core/AppVisibility.swift

@@ -0,0 +1,12 @@
+import Foundation
+import Combine
+
+/// Tracks whether the app is in the foreground, ported from `platform/AppVisibility.kt`.
+@MainActor
+final class AppVisibility: ObservableObject {
+	@Published private(set) var isForeground: Bool = true
+
+	func setForeground(_ value: Bool) {
+		isForeground = value
+	}
+}