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/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
+}