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/Features/App/AppModel.swift

@@ -0,0 +1,44 @@
+import Foundation
+import Combine
+
+/// Top-level app state, ported from `feature/app/AppViewModel.kt`. Initializes the
+/// core on launch and tracks the selected destination + theme.
+@MainActor
+final class AppModel: ObservableObject {
+	@Published private(set) var destination: AppDestination = .send
+	@Published private(set) var themeMode: ThemeMode = .system
+
+	private let environment: PlatformEnvironment
+	private let repository: CoreRepository
+	private let messages: UiMessageController
+	private var cancellables = Set<AnyCancellable>()
+
+	init(
+		environment: PlatformEnvironment,
+		repository: CoreRepository,
+		preferences: AppPreferencesRepository,
+		messages: UiMessageController
+	) {
+		self.environment = environment
+		self.repository = repository
+		self.messages = messages
+
+		AppLogger.info("lifecycle", "app started", ["platform": environment.name])
+
+		Task {
+			let result = await repository.initialize(appDataDir: environment.defaultCoreDataDir)
+			if case .failure(let error) = result { messages.error(error) }
+		}
+
+		preferences.$preferences
+			.map(\.themeMode)
+			.removeDuplicates()
+			.sink { [weak self] mode in self?.themeMode = mode }
+			.store(in: &cancellables)
+	}
+
+	func selectDestination(_ destination: AppDestination) {
+		guard destination != self.destination else { return }
+		self.destination = destination
+	}
+}