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/PlatformPickers.swift

@@ -0,0 +1,119 @@
+import SwiftUI
+import UniformTypeIdentifiers
+
+/// Root-level pickers that are NOT triggered from inside a sheet. Currently just
+/// the receive-folder picker (Settings is presented in the tab's navigation
+/// stack, not a sheet, so presenting from the root works).
+struct PlatformPickers: ViewModifier {
+	@ObservedObject var settingsModel: SettingsModel
+
+	func body(content: Content) -> some View {
+		content
+			.fileImporter(
+				isPresented: Binding(get: { settingsModel.pendingReceiveFolderPick }, set: { settingsModel.pendingReceiveFolderPick = $0 }),
+				allowedContentTypes: [.folder],
+				allowsMultipleSelection: false
+			) { result in
+				switch result {
+				case .success(let urls):
+					guard let url = urls.first else { return }
+					settingsModel.onReceiveFolderPicked(PickerSupport.receiveFolder(from: url))
+				case .failure(let error):
+					if !error.isUserCancellation { settingsModel.onReceiveFolderPickFailed(error.technicalDetail) }
+				}
+			}
+	}
+}
+
+/// Send file/folder pickers. Must be attached to the composer view so the picker
+/// presents from the composer's sheet, not the already-presenting root controller.
+struct SendPickers: ViewModifier {
+	@ObservedObject var model: SendModel
+
+	func body(content: Content) -> some View {
+		content
+			.fileImporter(
+				isPresented: Binding(get: { model.pendingFilePick }, set: { model.pendingFilePick = $0 }),
+				allowedContentTypes: [.item],
+				allowsMultipleSelection: true
+			) { result in
+				handleShareSelection(result, isDirectory: false)
+			}
+			.fileImporter(
+				isPresented: Binding(get: { model.pendingFolderPick }, set: { model.pendingFolderPick = $0 }),
+				allowedContentTypes: [.folder],
+				allowsMultipleSelection: false
+			) { result in
+				handleShareSelection(result, isDirectory: true)
+			}
+	}
+
+	private func handleShareSelection(_ result: Result<[URL], Error>, isDirectory: Bool) {
+		switch result {
+		case .success(let urls):
+			let files = urls.compactMap { PickerSupport.pickedFile(from: $0, isDirectory: isDirectory) }
+			if files.isEmpty {
+				model.onFilePickFailed("The selected document could not be opened")
+			} else {
+				model.onFilesPicked(files)
+			}
+		case .failure(let error):
+			if !error.isUserCancellation { model.onFilePickFailed(error.technicalDetail) }
+		}
+	}
+}
+
+enum PickerSupport {
+	static func receiveFolder(from url: URL) -> ReceiveFolder {
+		#if os(iOS)
+		// External receive folders on iOS use security-scoped URLs; the core holds
+		// access while streaming. Store the URL string.
+		return ReceiveFolder(kind: .iosSecurityScopedUrl, value: url.absoluteString, displayName: url.lastPathComponent)
+		#else
+		return ReceiveFolder(kind: .fileSystemPath, value: url.path, displayName: url.lastPathComponent)
+		#endif
+	}
+
+	static func pickedFile(from url: URL, isDirectory: Bool) -> PickedShareFile? {
+		let started = url.startAccessingSecurityScopedResource()
+		defer { if started { url.stopAccessingSecurityScopedResource() } }
+
+		#if os(iOS)
+		// Copy into a temporary sandbox location so the core can read the file
+		// after the picker/security scope ends. Folders are passed by path.
+		if isDirectory {
+			return PickedShareFile(value: url.path, displayName: url.lastPathComponent, isDirectory: true)
+		}
+		let tempDir = FileManager.default.temporaryDirectory
+			.appendingPathComponent("share-\(UUID().uuidString)", isDirectory: true)
+		try? FileManager.default.createDirectory(at: tempDir, withIntermediateDirectories: true)
+		let dest = tempDir.appendingPathComponent(url.lastPathComponent)
+		do {
+			try FileManager.default.copyItem(at: url, to: dest)
+		} catch {
+			return nil
+		}
+		let size = (try? dest.resourceValues(forKeys: [.fileSizeKey]))?.fileSize.map { UInt64($0) }
+		return PickedShareFile(
+			value: dest.path, displayName: url.lastPathComponent, sizeBytes: size,
+			isTemporaryCopy: true, isDirectory: false
+		)
+		#else
+		let size = isDirectory ? nil : (try? url.resourceValues(forKeys: [.fileSizeKey]))?.fileSize.map { UInt64($0) }
+		return PickedShareFile(
+			value: url.path, displayName: url.lastPathComponent, sizeBytes: size,
+			isTemporaryCopy: false, isDirectory: isDirectory
+		)
+		#endif
+	}
+}
+
+extension View {
+	func platformPickers(settingsModel: SettingsModel) -> some View {
+		modifier(PlatformPickers(settingsModel: settingsModel))
+	}
+
+	func sendPickers(model: SendModel) -> some View {
+		modifier(SendPickers(model: model))
+	}
+}