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/SnackbarHost.swift

@@ -0,0 +1,79 @@
+import SwiftUI
+
+/// Bottom toast host driven by `UiMessageController`, ported from
+/// `ui/feedback/VniDropSnackbarHost.kt`. Tone drives the accent color; errors get
+/// a longer display duration.
+struct SnackbarHost: View {
+	@ObservedObject var controller: UiMessageController
+	@State private var dismissTask: Task<Void, Never>?
+
+	var body: some View {
+		VStack {
+			Spacer()
+			if let message = controller.current {
+				content(for: message)
+					.frame(maxWidth: 520)
+					.background(.regularMaterial, in: RoundedRectangle(cornerRadius: 14))
+					.overlay(RoundedRectangle(cornerRadius: 14).stroke(.gray.opacity(0.25), lineWidth: 0.5))
+					.shadow(color: .black.opacity(0.15), radius: 8, y: 2)
+					.padding(.horizontal, 16)
+					.padding(.bottom, 8)
+					.transition(.move(edge: .bottom).combined(with: .opacity))
+					.id(message.id)
+					.onAppear { scheduleDismiss(for: message) }
+			}
+		}
+		.animation(.easeInOut(duration: 0.2), value: controller.current?.id)
+	}
+
+	@ViewBuilder
+	private func content(for message: UiMessage) -> some View {
+		let accent: Color = {
+			switch message.tone {
+			case .info: return VniDropColors.brandPurple
+			case .success: return .green
+			case .warning: return .orange
+			case .error: return .red
+			}
+		}()
+		HStack(alignment: .center, spacing: 8) {
+			Circle().fill(accent).frame(width: 8, height: 8)
+			Text(message.text.resolved())
+				.font(.subheadline)
+				.frame(maxWidth: .infinity, alignment: .leading)
+				.padding(.vertical, 10)
+			if let actionLabel = message.actionLabel {
+				Button(action: {
+					message.onAction?()
+					dismiss()
+				}) {
+					Text(actionLabel.resolved()).fontWeight(.semibold)
+				}
+				.buttonStyle(.borderless)
+			}
+			Button(action: dismiss) {
+				Image(systemName: "xmark")
+					.font(.footnote.weight(.semibold))
+					.foregroundStyle(.secondary)
+					.frame(width: 36, height: 36)
+			}
+			.buttonStyle(.plain)
+		}
+		.padding(.leading, 16)
+		.padding(.trailing, 4)
+	}
+
+	private func scheduleDismiss(for message: UiMessage) {
+		dismissTask?.cancel()
+		let seconds: UInt64 = message.tone == .error ? 6 : 4
+		dismissTask = Task {
+			try? await Task.sleep(nanoseconds: seconds * 1_000_000_000)
+			if !Task.isCancelled { controller.advance() }
+		}
+	}
+
+	private func dismiss() {
+		dismissTask?.cancel()
+		controller.advance()
+	}
+}

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))
+	}
+}

apple/VniDrop/UI/Feedback/UserFacingError.swift

@@ -0,0 +1,126 @@
+import Foundation
+import VnidropCore
+
+/// Maps technical failures to stable, user-facing catalog keys. Ported from
+/// `ui/feedback/UserFacingError.kt`. Never exposes raw `reason=` blobs.
+extension Error {
+	func toUiText() -> UiText {
+		if let vni = self as? VnidropError {
+			switch vni {
+			case .Ticket:
+				return .resource("error_invalid_ticket")
+			case .Permission:
+				return .resource("error_permission")
+			case .Filesystem:
+				return .resource("error_filesystem")
+			case .Transfer(let reason):
+				return transferUiText(reason)
+			case .Repository:
+				return .resource("error_repository")
+			case .Initialization(let reason):
+				return initializationUiText(reason)
+			case .Internal(let reason):
+				return reasonHints(reason) ?? .resource("error_generic")
+			}
+		}
+		return reasonHints(technicalDetail) ?? .resource("error_generic")
+	}
+
+	/// True when the user intentionally backed out of a flow.
+	var isUserCancellation: Bool {
+		let haystack = technicalDetail.lowercased()
+		if haystack.isEmpty {
+			// URLError / CocoaError cancellation without a message.
+			if let urlError = self as? URLError, urlError.code == .cancelled { return true }
+			return (self as NSError).code == NSUserCancelledError
+		}
+		return haystack.contains("cancelled")
+			|| haystack.contains("canceled")
+			|| haystack.contains("user cancelled")
+			|| haystack.contains("user canceled")
+	}
+
+	/// Prefers a `VnidropError` reason; else the localized description.
+	var technicalDetail: String {
+		if let vni = self as? VnidropError {
+			switch vni {
+			case .Initialization(let r), .Ticket(let r), .Filesystem(let r),
+				 .Transfer(let r), .Permission(let r), .Repository(let r), .Internal(let r):
+				return r
+			}
+		}
+		return (self as? LocalizedError)?.errorDescription ?? (self as NSError).localizedDescription
+	}
+}
+
+private func transferUiText(_ reason: String) -> UiText {
+	let detail = reason.lowercased()
+	if detail.contains("refused") || detail.contains("denied") || detail.contains("not approved") {
+		return .resource("error_permission")
+	}
+	return .resource("error_transfer")
+}
+
+private func initializationUiText(_ reason: String) -> UiText {
+	let detail = reason.lowercased()
+	if detail.contains("native") && detail.contains("library") {
+		return .resource("error_missing_native_library")
+	}
+	if detail.contains("socket") || detail.contains("bind") {
+		return .resource("error_socket_bind")
+	}
+	return .resource("error_initialization")
+}
+
+private func reasonHints(_ detailRaw: String) -> UiText? {
+	let detail = detailRaw.lowercased()
+	if detail.isEmpty { return nil }
+
+	if detail.contains("still starting") || detail.contains("starting up") {
+		return .resource("error_starting_up")
+	}
+	if detail.contains("empty") && (detail.contains("invitation") || detail.contains("ticket") || detail.contains("qr")) {
+		return .resource("error_invitation_empty")
+	}
+	if detail.contains("select at least one") || detail.contains("no files found") {
+		return .resource("error_share_empty")
+	}
+	if detail.contains("camera") {
+		return .resource("error_camera")
+	}
+	if detail.contains("nfc") || detail.contains("ndef")
+		|| (detail.contains("read-only") && detail.contains("tag"))
+		|| detail.contains("tag is too small") || detail.contains("no nfc tag") {
+		return .resource("error_nfc")
+	}
+	if detail.contains("native") && detail.contains("library") {
+		return .resource("error_missing_native_library")
+	}
+	if detail.contains("socket") || detail.contains("bind") {
+		return .resource("error_socket_bind")
+	}
+	if detail.contains("device information") || detail.contains("device info") {
+		return .resource("error_device_info")
+	}
+	if detail.contains("refused") || detail.contains("denied") || detail.contains("permission")
+		|| detail.contains("not approved") || detail.contains("waiting for approval") {
+		return .resource("error_permission")
+	}
+	if detail.contains("invalid ticket") || detail.contains("ticket error")
+		|| detail.contains("could not be read") || detail.contains("malformed")
+		|| detail.contains("invitation could not be opened") {
+		return .resource("error_invalid_ticket")
+	}
+	if detail.contains("selected")
+		&& (detail.contains("file") || detail.contains("folder") || detail.contains("document") || detail.contains("open")) {
+		return .resource("error_selection_failed")
+	}
+	if detail.contains("could not open the selected") || detail.contains("could not open selected") {
+		return .resource("error_selection_failed")
+	}
+	if detail.contains("document picker") || detail.contains("folder picker") || detail.contains("file descriptor")
+		|| detail.contains("view controller") {
+		return .resource("error_selection_failed")
+	}
+	return nil
+}