iOS: export/import stem playlists and sound settings

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

ios/Stemify/Core/Backup.swift

@@ -0,0 +1,84 @@
+import Foundation
+import SwiftUI
+import UniformTypeIdentifiers
+
+/// What an export file holds: stem playlists (with each song's mix) and the non-secret sound settings.
+/// Plex and stem server tokens and URLs are never included.
+struct StemifyExport: Codable {
+    static let format = "stemify-export"
+    static let version = 1
+
+    struct Prefs: Codable {
+        var stemQuality: String?
+        var keepStemMix: Bool?
+        var karaokeVocals: Int?
+        var karaokeSoftDb: Int?
+        var playbackSpeed: Double?
+        var downloadsWifiOnly: Bool?
+    }
+
+    var format = StemifyExport.format
+    var version = StemifyExport.version
+    var exported = Date()
+    var playlists: [StemPlaylist]
+    var prefs: Prefs
+
+    @MainActor
+    static func capture(_ lists: StemPlaylists, _ s: Settings) -> StemifyExport {
+        StemifyExport(playlists: lists.lists, prefs: Prefs(
+            stemQuality: s.stemQuality.rawValue, keepStemMix: s.keepStemMix, karaokeVocals: s.karaokeVocals,
+            karaokeSoftDb: s.karaokeSoftDb, playbackSpeed: s.playbackSpeed, downloadsWifiOnly: s.downloadsWifiOnly))
+    }
+
+    func encoded() throws -> Data {
+        let e = JSONEncoder()
+        e.outputFormatting = [.prettyPrinted, .sortedKeys]
+        e.dateEncodingStrategy = .iso8601
+        return try e.encode(self)
+    }
+
+    static func decode(_ data: Data) throws -> StemifyExport {
+        let d = JSONDecoder()
+        d.dateDecodingStrategy = .iso8601
+        let x = try d.decode(StemifyExport.self, from: data)
+        guard x.format == Self.format else { throw BackupError.notAnExport }
+        guard x.version <= Self.version else { throw BackupError.newerVersion }
+        return x
+    }
+
+    @MainActor
+    func apply(to s: Settings) {
+        if let v = prefs.stemQuality.flatMap(StemQuality.init(rawValue:)) { s.stemQuality = v }
+        if let v = prefs.keepStemMix { s.keepStemMix = v }
+        if let v = prefs.karaokeVocals { s.karaokeVocals = v }
+        if let v = prefs.karaokeSoftDb { s.karaokeSoftDb = v }
+        if let v = prefs.playbackSpeed { s.playbackSpeed = v }
+        if let v = prefs.downloadsWifiOnly { s.downloadsWifiOnly = v }
+    }
+}
+
+enum BackupError: LocalizedError {
+    case notAnExport, newerVersion
+    var errorDescription: String? {
+        switch self {
+        case .notAnExport: "That file isn't a Stemify export."
+        case .newerVersion: "That export is from a newer version of Stemify."
+        }
+    }
+}
+
+/// Where an export goes and comes from. The Files sheet covers local storage and iCloud Drive;
+/// a direct iCloud (ubiquity container / CloudKit) backend would be another conformer.
+protocol BackupDestination {
+    func write(_ data: Data, name: String) async throws
+    func read() async throws -> Data
+}
+
+/// The export as a document for fileExporter, which also reaches iCloud Drive through Files.
+struct ExportDocument: FileDocument {
+    static var readableContentTypes: [UTType] { [.json] }
+    var data: Data
+    init(data: Data = Data()) { self.data = data }
+    init(configuration: ReadConfiguration) throws { data = configuration.file.regularFileContents ?? Data() }
+    func fileWrapper(configuration: WriteConfiguration) throws -> FileWrapper { FileWrapper(regularFileWithContents: data) }
+}