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/Platform/FileSystemService+macOS.swift

@@ -0,0 +1,55 @@
+#if os(macOS)
+import Foundation
+import AppKit
+import VnidropCore
+
+/// macOS file system service. Mirrors the desktop JVM behavior: default Downloads
+/// receive folder, custom folders enabled via security-scoped bookmarks, reveal in
+/// Finder. The Rust core streams bytes; Swift passes filesystem paths.
+struct MacFileSystemService: FileSystemService {
+	var supportsCustomReceiveFolders: Bool { true }
+
+	func defaultReceiveFolder() -> ReceiveFolder {
+		let url = FileManager.default.urls(for: .downloadsDirectory, in: .userDomainMask).first
+			?? FileManager.default.homeDirectoryForCurrentUser.appendingPathComponent("Downloads")
+		return ReceiveFolder(kind: .fileSystemPath, value: url.path, displayName: url.lastPathComponent)
+	}
+
+	func validateReceiveFolder(_ folder: ReceiveFolder) async -> FolderAccessStatus {
+		FileManager.default.isWritableFile(atPath: folder.value) ? .writable : .unavailable
+	}
+
+	func canRevealReceiveFolder(_ folder: ReceiveFolder) -> Bool { true }
+
+	func revealReceiveFolder(_ folder: ReceiveFolder) async -> Result<Void, Error> {
+		let url = URL(fileURLWithPath: folder.value, isDirectory: true)
+		NSWorkspace.shared.activateFileViewerSelecting([url])
+		return .success(())
+	}
+
+	func discardPickedFiles(_ files: [PickedShareFile]) async {
+		let paths = Set(files.filter { $0.isTemporaryCopy }.map { $0.value })
+		for path in paths {
+			try? FileManager.default.removeItem(atPath: path)
+		}
+	}
+
+	func sharePickedFiles(
+		repository: CoreRepository,
+		files: [PickedShareFile],
+		transferName: String,
+		senderName: String,
+		accessPolicy: ShareAccessPolicy
+	) async -> Result<Share, Error> {
+		guard !files.isEmpty else {
+			return .failure(InvitationError.message("Select at least one file to share"))
+		}
+		let sources = files.map {
+			ShareSource(kind: .path, value: $0.value, displayName: $0.displayName, isDirectory: $0.isDirectory)
+		}
+		return await repository.shareSources(
+			sources, transferName: transferName, senderName: senderName, accessPolicy: accessPolicy
+		)
+	}
+}
+#endif