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+iOS.swift

@@ -0,0 +1,98 @@
+#if os(iOS)
+import Foundation
+import UIKit
+import VnidropCore
+
+/// iOS file system service, ported from `FileSystemService.ios.kt`.
+/// App-owned Documents is the fixed receive folder; custom folders are not
+/// supported because raw external picker URLs do not survive relaunch.
+struct IosFileSystemService: FileSystemService {
+	var supportsCustomReceiveFolders: Bool { false }
+
+	func defaultReceiveFolder() -> ReceiveFolder {
+		let path = FileManager.default
+			.urls(for: .documentDirectory, in: .userDomainMask)
+			.first?.path ?? ""
+		return ReceiveFolder(kind: .fileSystemPath, value: path, displayName: "Documents")
+	}
+
+	func validateReceiveFolder(_ folder: ReceiveFolder) async -> FolderAccessStatus {
+		switch folder.kind {
+		case .fileSystemPath:
+			return FileManager.default.isWritableFile(atPath: folder.value) ? .writable : .unavailable
+		case .iosSecurityScopedUrl:
+			return validateSecurityScopedUrl(folder.value)
+		}
+	}
+
+	func canRevealReceiveFolder(_ folder: ReceiveFolder) -> Bool {
+		folder.kind == .fileSystemPath
+			&& folder.value.trimmingTrailingSlash == defaultReceiveFolder().value.trimmingTrailingSlash
+	}
+
+	func revealReceiveFolder(_ folder: ReceiveFolder) async -> Result<Void, Error> {
+		guard canRevealReceiveFolder(folder) else {
+			return .failure(InvitationError.message("The receive folder is not VniDrop Documents"))
+		}
+		guard let url = URL(string: "shareddocuments://\(folder.value)") else {
+			return .failure(InvitationError.message("The Files location URL is unavailable"))
+		}
+		let opened = await withCheckedContinuation { continuation in
+			DispatchQueue.main.async {
+				UIApplication.shared.open(url, options: [:]) { success in
+					continuation.resume(returning: success)
+				}
+			}
+		}
+		return opened ? .success(()) : .failure(InvitationError.message("Could not open VniDrop Documents in Files"))
+	}
+
+	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 { $0.toIosShareSource() }
+		return await repository.shareSources(
+			sources, transferName: transferName, senderName: senderName, accessPolicy: accessPolicy
+		)
+	}
+
+	private func validateSecurityScopedUrl(_ value: String) -> FolderAccessStatus {
+		let url = URL(string: value) ?? URL(fileURLWithPath: value)
+		let started = url.startAccessingSecurityScopedResource()
+		defer { if started { url.stopAccessingSecurityScopedResource() } }
+		if FileManager.default.isWritableFile(atPath: url.path) {
+			return .writable
+		}
+		return .permissionRequired
+	}
+}
+
+extension PickedShareFile {
+	/// iOS shares by filesystem path (from `asCopy` picker temp files).
+	func toIosShareSource() -> ShareSource {
+		ShareSource(kind: .path, value: value, displayName: displayName, isDirectory: isDirectory)
+	}
+}
+
+private extension String {
+	var trimmingTrailingSlash: String {
+		var s = self
+		while s.hasSuffix("/") { s.removeLast() }
+		return s
+	}
+}
+#endif