feat(playlist): client-side sort and search, with the control in the toolbar

Sources/Kaset/Models/PlaylistSortOrder.swift

@@ -0,0 +1,36 @@
+import Foundation
+
+// MARK: - PlaylistSortKey
+
+/// A sort key for the playlist detail track list.
+enum PlaylistSortKey: String, CaseIterable, Identifiable {
+    case original
+    case title
+    case artist
+    case duration
+    case album
+
+    var id: String {
+        self.rawValue
+    }
+
+    var displayName: String {
+        switch self {
+        case .original: String(localized: "Original Order")
+        case .title: String(localized: "Title")
+        case .artist: String(localized: "Artist")
+        case .duration: String(localized: "Duration")
+        case .album: String(localized: "Album")
+        }
+    }
+}
+
+// MARK: - PlaylistSortOrder
+
+/// A sort key plus direction. `.original` preserves the server-provided order.
+struct PlaylistSortOrder: Equatable {
+    var key: PlaylistSortKey
+    var ascending: Bool
+
+    static let `default` = PlaylistSortOrder(key: .original, ascending: true)
+}

Sources/Kaset/Models/Song.swift

@@ -154,6 +154,16 @@ struct Song: Identifiable, Codable, Hashable {
         self.artists.map(\.name).joined(separator: ", ")
     }
 
+    /// Stable per-occurrence identity, also safe as a SwiftUI `ForEach` id — position isn't
+    /// identity in a sortable list. The blank guard and namespacing prevent collisions: a
+    /// blank set id would collapse every such row onto one identity.
+    var rowIdentity: String {
+        if let setVideoId = self.playlistSetVideoId, !setVideoId.isEmpty {
+            return "set:\(setVideoId)"
+        }
+        return "video:\(self.videoId)"
+    }
+
     /// Formatted duration string (e.g., "3:45").
     var durationDisplay: String {
         guard let duration else { return "--:--" }