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/FileSystemService.swift

@@ -0,0 +1,58 @@
+import Foundation
+import VnidropCore
+
+/// A file/folder selected for sharing, ported from `PickedShareFile` in
+/// `core/FilePicker.kt`.
+struct PickedShareFile: Equatable, Identifiable {
+	let value: String
+	let displayName: String
+	var sizeBytes: UInt64? = nil
+	var thumbnailData: Data? = nil
+	/// App-owned picker copy that may be deleted after import or abandonment.
+	var isTemporaryCopy: Bool = false
+	/// When true, `value` is a directory (path or security-scoped folder URL).
+	var isDirectory: Bool = false
+
+	var id: String { value }
+}
+
+/// Receive-destination and share-source platform bridge, ported from
+/// `core/FileSystemService.kt` and its iOS/desktop actuals.
+protocol FileSystemService {
+	var supportsCustomReceiveFolders: Bool { get }
+
+	func defaultReceiveFolder() -> ReceiveFolder
+	func effectiveReceiveFolder(_ configured: ReceiveFolder) -> ReceiveFolder
+	func validateReceiveFolder(_ folder: ReceiveFolder) async -> FolderAccessStatus
+	func canRevealReceiveFolder(_ folder: ReceiveFolder) -> Bool
+	func revealReceiveFolder(_ folder: ReceiveFolder) async -> Result<Void, Error>
+	/// Releases only app-owned picker copies; never deletes original user sources.
+	func discardPickedFiles(_ files: [PickedShareFile]) async
+	func sharePickedFiles(
+		repository: CoreRepository,
+		files: [PickedShareFile],
+		transferName: String,
+		senderName: String,
+		accessPolicy: ShareAccessPolicy
+	) async -> Result<Share, Error>
+}
+
+extension FileSystemService {
+	var supportsCustomReceiveFolders: Bool { true }
+
+	func effectiveReceiveFolder(_ configured: ReceiveFolder) -> ReceiveFolder {
+		supportsCustomReceiveFolders ? configured : defaultReceiveFolder()
+	}
+
+	func canRevealReceiveFolder(_ folder: ReceiveFolder) -> Bool { false }
+
+	func revealReceiveFolder(_ folder: ReceiveFolder) async -> Result<Void, Error> {
+		.failure(InvitationError.message("Revealing the receive folder is not supported"))
+	}
+
+	func discardPickedFiles(_ files: [PickedShareFile]) async {}
+}
+
+extension ReceiveFolder {
+	var isFileSystemPath: Bool { kind == .fileSystemPath }
+}