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/AppPreferences.swift

@@ -0,0 +1,135 @@
+import Foundation
+import Combine
+
+/// Receive-destination descriptor, ported from `core/FileSystemService.kt`.
+enum ReceiveFolderKind: String, Codable {
+	case fileSystemPath
+	case iosSecurityScopedUrl
+}
+
+struct ReceiveFolder: Equatable, Codable {
+	let kind: ReceiveFolderKind
+	let value: String
+	let displayName: String
+}
+
+enum FolderAccessStatus {
+	case writable
+	case permissionRequired
+	case unavailable
+}
+
+/// Persisted app preferences, ported from `preferences/AppPreferencesRepository.kt`.
+/// Backed by `UserDefaults` instead of DataStore; keys and semantics match.
+struct AppPreferences: Equatable {
+	var username: String
+	var receiveFolder: ReceiveFolder
+	var themeMode: ThemeMode
+	var notificationsEnabled: Bool
+	var diagnosticsEnabled: Bool
+	var diagnosticsInstallId: String
+}
+
+struct AppPreferencesDefaults {
+	let username: String
+	let receiveFolder: ReceiveFolder
+	let themeMode: ThemeMode
+	var notificationsEnabled: Bool = false
+	var diagnosticsEnabled: Bool = false
+}
+
+@MainActor
+final class AppPreferencesRepository: ObservableObject {
+	@Published private(set) var preferences: AppPreferences
+
+	private let defaults: UserDefaults
+	private let fallback: AppPreferencesDefaults
+
+	private enum Key {
+		static let username = "username"
+		static let receiveFolderKind = "receive_folder_kind"
+		static let receiveFolderValue = "receive_folder_value"
+		static let receiveFolderDisplayName = "receive_folder_display_name"
+		static let themeMode = "theme_mode"
+		static let notificationsEnabled = "notifications_enabled"
+		static let diagnosticsEnabled = "diagnostics_enabled"
+		static let diagnosticsInstallId = "diagnostics_install_id"
+	}
+
+	init(defaults: UserDefaults = .standard, fallback: AppPreferencesDefaults) {
+		self.defaults = defaults
+		self.fallback = fallback
+		self.preferences = Self.read(from: defaults, fallback: fallback)
+	}
+
+	private static func read(from defaults: UserDefaults, fallback: AppPreferencesDefaults) -> AppPreferences {
+		let username = (defaults.string(forKey: Key.username)).flatMap { $0.isEmpty ? nil : $0 } ?? fallback.username
+		let folder = resolveReceiveFolder(defaults, fallback: fallback.receiveFolder)
+		let themeMode = defaults.string(forKey: Key.themeMode).flatMap(ThemeMode.init(rawValue:)) ?? fallback.themeMode
+		let notifications = defaults.object(forKey: Key.notificationsEnabled) as? Bool ?? fallback.notificationsEnabled
+		let diagnostics = defaults.object(forKey: Key.diagnosticsEnabled) as? Bool ?? fallback.diagnosticsEnabled
+		let installId = defaults.string(forKey: Key.diagnosticsInstallId) ?? ""
+		return AppPreferences(
+			username: username,
+			receiveFolder: folder,
+			themeMode: themeMode,
+			notificationsEnabled: notifications,
+			diagnosticsEnabled: diagnostics,
+			diagnosticsInstallId: installId
+		)
+	}
+
+	private static func resolveReceiveFolder(_ defaults: UserDefaults, fallback: ReceiveFolder) -> ReceiveFolder {
+		let kind = defaults.string(forKey: Key.receiveFolderKind)
+			.flatMap(ReceiveFolderKind.init(rawValue:)) ?? fallback.kind
+		let value = defaults.string(forKey: Key.receiveFolderValue).flatMap { $0.isEmpty ? nil : $0 } ?? fallback.value
+		let displayName = defaults.string(forKey: Key.receiveFolderDisplayName)
+			.flatMap { $0.isEmpty ? nil : $0 } ?? fallback.displayName
+		return ReceiveFolder(kind: kind, value: value, displayName: displayName)
+	}
+
+	private func reload() {
+		preferences = Self.read(from: defaults, fallback: fallback)
+	}
+
+	func setUsername(_ username: String) {
+		defaults.set(username.trimmingCharacters(in: .whitespacesAndNewlines), forKey: Key.username)
+		reload()
+	}
+
+	func setReceiveFolder(_ folder: ReceiveFolder) {
+		defaults.set(folder.kind.rawValue, forKey: Key.receiveFolderKind)
+		defaults.set(folder.value, forKey: Key.receiveFolderValue)
+		defaults.set(folder.displayName, forKey: Key.receiveFolderDisplayName)
+		reload()
+	}
+
+	func resetReceiveFolder() {
+		setReceiveFolder(fallback.receiveFolder)
+	}
+
+	func setThemeMode(_ mode: ThemeMode) {
+		defaults.set(mode.rawValue, forKey: Key.themeMode)
+		reload()
+	}
+
+	func setNotificationsEnabled(_ enabled: Bool) {
+		defaults.set(enabled, forKey: Key.notificationsEnabled)
+		reload()
+	}
+
+	func setDiagnosticsEnabled(_ enabled: Bool) {
+		defaults.set(enabled, forKey: Key.diagnosticsEnabled)
+		reload()
+	}
+
+	@discardableResult
+	func ensureDiagnosticsInstallId() -> String {
+		let existing = preferences.diagnosticsInstallId
+		if !existing.isEmpty { return existing }
+		let created = UUID().uuidString
+		defaults.set(created, forKey: Key.diagnosticsInstallId)
+		reload()
+		return created
+	}
+}