iOS 1.0.8: stem playlists, karaoke softness slider in lyrics

- Playlists tab splits into Plex and Stem. Plex playlists are unchanged and
  now show their custom poster when one is set.
- Stem playlists live on the device; each song keeps its own mix. Add from
  any song or collection menu, or copy a whole Plex playlist.
- Queue items can carry a mix, which wins over the song's saved mix.
- Softness slider under the Vocals picker in lyrics when set to Soft.

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

ios/Stemify/Playback/StemPlaylists.swift

@@ -0,0 +1,86 @@
+import Foundation
+import Observation
+
+/// One song in a stem playlist, saved with the mix it plays with.
+struct StemPlaylistItem: Codable, Hashable, Identifiable {
+    var uid = UUID().uuidString
+    var track: Track
+    var mix: [String: StemSetting]
+    var id: String { uid }
+}
+
+struct StemPlaylist: Codable, Hashable, Identifiable {
+    var id = UUID().uuidString
+    var title: String
+    var items: [StemPlaylistItem] = []
+    var created = Date()
+    var updated = Date()
+
+    var tracks: [Track] { items.map(\.track) }
+    var mixes: [[String: StemSetting]] { items.map(\.mix) }
+    var thumb: String? { items.lazy.compactMap(\.track.thumb).first }
+    var durationMs: Int64 { items.reduce(0) { $0 + $1.track.durationMs } }
+}
+
+/// Playlists that live on this device only: each song keeps its own stem mix (instrumentals, a cappellas…).
+/// Plex playlists can't hold a mix, so these are separate.
+@MainActor @Observable
+final class StemPlaylists {
+    private(set) var lists: [StemPlaylist] = []
+    @ObservationIgnored private let file: URL
+
+    /// Every stem a song can have, for naming saved mixes without loading the song.
+    static let allStems = ["vocals", "drums", "bass", "guitar", "piano", "other"]
+
+    init(dir: URL) {
+        file = dir.appendingPathComponent("stem-playlists.json")
+        lists = (try? JSONDecoder().decode([StemPlaylist].self, from: Data(contentsOf: file))) ?? []
+    }
+
+    func list(_ id: String) -> StemPlaylist? { lists.first { $0.id == id } }
+
+    @discardableResult
+    func create(_ title: String, tracks: [Track] = [], mix: [String: StemSetting] = [:]) -> StemPlaylist {
+        let pl = StemPlaylist(title: title, items: tracks.map { StemPlaylistItem(track: $0, mix: MixPresets.normalized(mix)) })
+        lists.insert(pl, at: 0)
+        save()
+        return pl
+    }
+
+    func add(_ id: String, tracks: [Track], mix: [String: StemSetting]) {
+        edit(id) { $0.items += tracks.map { StemPlaylistItem(track: $0, mix: MixPresets.normalized(mix)) } }
+    }
+
+    func setItems(_ id: String, _ items: [StemPlaylistItem]) { edit(id) { $0.items = items } }
+    func remove(_ id: String, item: String) { edit(id) { $0.items.removeAll { $0.uid == item } } }
+    func setMix(_ id: String, item: String, _ mix: [String: StemSetting]) {
+        edit(id) { pl in if let i = pl.items.firstIndex(where: { $0.uid == item }) { pl.items[i].mix = MixPresets.normalized(mix) } }
+    }
+    func rename(_ id: String, _ title: String) { edit(id) { $0.title = title } }
+
+    func delete(_ id: String) {
+        lists.removeAll { $0.id == id }
+        save()
+    }
+
+    /// "Instrumental", "A Cappella"… or "Custom Mix" for a hand-made one.
+    static func label(_ mix: [String: StemSetting]) -> String {
+        if MixPresets.normalized(mix).isEmpty { return MixPresets.full.label }
+        if MixPresets.current(allStems, mix) == MixPresets.karaoke { return "Instrumental" }
+        return MixPresets.current(allStems, mix)?.label ?? "Custom Mix"
+    }
+
+    private func edit(_ id: String, _ change: (inout StemPlaylist) -> Void) {
+        guard let i = lists.firstIndex(where: { $0.id == id }) else { return }
+        change(&lists[i])
+        lists[i].updated = Date()
+        save()
+    }
+
+    private func save() {
+        let all = lists, file = file
+        DispatchQueue.global(qos: .utility).async {
+            if let data = try? JSONEncoder().encode(all) { try? data.write(to: file, options: .atomic) }
+        }
+    }
+}