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/Feedback/UiMessage.swift

@@ -0,0 +1,78 @@
+import Foundation
+import Combine
+
+/// A localizable UI string: either a catalog key or dynamic text, ported from
+/// `UiText` in `ui/feedback/UiMessageController.kt`.
+enum UiText: Equatable {
+	case resource(String)   // Localizable.xcstrings key
+	case dynamic(String)
+
+	/// Resolves to display text. Keys go through the string catalog.
+	func resolved() -> String {
+		switch self {
+		case .dynamic(let value): return value
+		case .resource(let key): return String(localized: String.LocalizationValue(key))
+		}
+	}
+}
+
+enum UiMessageTone {
+	case info
+	case success
+	case warning
+	case error
+}
+
+struct UiMessage: Identifiable {
+	let id = UUID()
+	let text: UiText
+	var tone: UiMessageTone = .info
+	var actionLabel: UiText? = nil
+	var onAction: (() -> Void)? = nil
+}
+
+/// Queues user-facing messages (snackbars) and dismissal requests. Ported from
+/// `UiMessageController.kt`. Errors that are user cancellations are suppressed.
+@MainActor
+final class UiMessageController: ObservableObject {
+	@Published private(set) var current: UiMessage?
+	private var queue: [UiMessage] = []
+
+	func show(_ message: UiMessage) {
+		if current == nil {
+			current = message
+		} else {
+			queue.append(message)
+		}
+	}
+
+	@discardableResult
+	func tryShow(_ message: UiMessage) -> Bool {
+		show(message)
+		return true
+	}
+
+	/// Called by the host when the current message is dismissed or times out.
+	func advance() {
+		if queue.isEmpty {
+			current = nil
+		} else {
+			current = queue.removeFirst()
+		}
+	}
+
+	/// Surfaces a user-facing error. Logs the technical detail; suppresses user
+	/// cancellations. Mirrors `UiMessageController.error(Throwable)`.
+	func error(_ error: Error) {
+		if error.isUserCancellation {
+			AppLogger.info("ui", "suppressed user cancellation", ["detail": error.technicalDetail])
+			return
+		}
+		AppLogger.error("ui", "user-facing error", error)
+		show(UiMessage(text: error.toUiText(), tone: .error))
+	}
+
+	func error(_ text: UiText) {
+		show(UiMessage(text: text, tone: .error))
+	}
+}