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/ExternalInvitationController.swift

@@ -0,0 +1,66 @@
+import Foundation
+
+let vniDropInvitationMimeType = "application/vnd.vnidrop.transfer"
+let vniDropInvitationExtension = "vnd"
+let maxVniDropInvitationBytes = 64 * 1024
+
+/// Buffered ingress for invitation documents opened by the OS, ported from
+/// `ExternalInvitationController.kt`. Hosts can submit before the UI is attached
+/// during a cold launch; each document is consumed exactly once.
+@MainActor
+final class ExternalInvitationController: ObservableObject {
+	/// Emits validated (or failed) invitations. The app-level receive workflow
+	/// consumes each exactly once.
+	private var continuation: AsyncStream<Result<String, Error>>.Continuation?
+	lazy var invitations: AsyncStream<Result<String, Error>> = {
+		AsyncStream { continuation in
+			self.continuation = continuation
+		}
+	}()
+
+	func openInvitation(raw: String) {
+		continuation?.yield(validateInvitation(raw))
+	}
+
+	func reportOpenFailure(message: String) {
+		continuation?.yield(.failure(InvitationError.message(message)))
+	}
+}
+
+enum InvitationError: LocalizedError {
+	case empty
+	case tooLarge
+	case invalidEncoding
+	case message(String)
+
+	var errorDescription: String? {
+		switch self {
+		case .empty: return "The invitation is empty"
+		case .tooLarge: return "The invitation is too large"
+		case .invalidEncoding: return "The invitation is not valid text"
+		case .message(let m): return m
+		}
+	}
+}
+
+func validateInvitation(_ raw: String) -> Result<String, Error> {
+	if raw.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty {
+		return .failure(InvitationError.empty)
+	}
+	if raw.utf8.count > maxVniDropInvitationBytes {
+		return .failure(InvitationError.tooLarge)
+	}
+	return .success(raw)
+}
+
+/// Decode invitation document bytes as strict UTF-8, ported from
+/// `decodeInvitationBytes`. Rejects payloads that are not lossless UTF-8.
+func decodeInvitationBytes(_ bytes: Data) throws -> String {
+	guard !bytes.isEmpty else { throw InvitationError.empty }
+	guard bytes.count <= maxVniDropInvitationBytes else { throw InvitationError.tooLarge }
+	guard let text = String(data: bytes, encoding: .utf8), Data(text.utf8) == bytes else {
+		throw InvitationError.invalidEncoding
+	}
+	guard !text.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty else { throw InvitationError.empty }
+	return text
+}