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/PlaybackManager.swift

@@ -46,6 +46,8 @@ enum SourceKind { case None, Stems, Original }
 struct QueueItem: Codable, Hashable, Identifiable {
     var uid: Int64
     var track: Track
+    /// Set when the song came from a stem playlist; it plays with this mix instead of its saved one.
+    var mix: [String: StemSetting]?
     var id: Int64 { uid }
 }
 
@@ -204,16 +206,17 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
 
     // ---- queue --------------------------------------------------------------------------
 
-    private func items(_ tracks: [Track]) -> [QueueItem] {
-        tracks.map { t in
+    private func items(_ tracks: [Track], _ mixes: [[String: StemSetting]]? = nil) -> [QueueItem] {
+        tracks.enumerated().map { i, t in
             defer { nextUid += 1 }
-            return QueueItem(uid: nextUid, track: t)
+            return QueueItem(uid: nextUid, track: t, mix: mixes.flatMap { i < $0.count ? $0[i] : nil })
         }
     }
 
-    func playTracks(_ tracks: [Track], start: Int = 0, shuffle doShuffle: Bool = false) {
+    /// `mixes` lines up with `tracks`: each song plays with its own mix (stem playlists).
+    func playTracks(_ tracks: [Track], mixes: [[String: StemSetting]]? = nil, start: Int = 0, shuffle doShuffle: Bool = false) {
         if tracks.isEmpty { return }
-        let items = items(tracks)
+        let items = items(tracks, mixes)
         if doShuffle {
             unshuffled = items
             let first = start > 0 && start < items.count ? items[start] : items.randomElement()!
@@ -228,12 +231,12 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
         }
     }
 
-    func playNext(_ tracks: [Track]) { insert(index + 1, tracks) }
-    func addToQueue(_ tracks: [Track]) { insert(queue.count, tracks) }
+    func playNext(_ tracks: [Track], mixes: [[String: StemSetting]]? = nil) { insert(index + 1, tracks, mixes) }
+    func addToQueue(_ tracks: [Track], mixes: [[String: StemSetting]]? = nil) { insert(queue.count, tracks, mixes) }
 
-    private func insert(_ at: Int, _ tracks: [Track]) {
+    private func insert(_ at: Int, _ tracks: [Track], _ mixes: [[String: StemSetting]]? = nil) {
         if tracks.isEmpty { return }
-        let items = items(tracks)
+        let items = items(tracks, mixes)
         var q = queue
         q.insert(contentsOf: items, at: min(max(at, 0), q.count))
         queue = q
@@ -415,10 +418,11 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
         pushMix()
     }
 
-    /// The mix a song starts with: karaoke if that's on, else the song's saved mix, else cleared (or kept, per settings).
-    private func startMix(_ t: Track) -> [String: StemSetting] {
+    /// The mix a song starts with: karaoke if that's on, else its stem playlist mix, else the song's saved mix,
+    /// else cleared (or kept, per settings).
+    private func startMix(_ item: QueueItem) -> [String: StemSetting] {
         if karaoke { return karaokeMix }
-        return songMixes[t.ratingKey] ?? (settings.keepStemMix ? mix : [:])
+        return item.mix ?? songMixes[item.track.ratingKey] ?? (settings.keepStemMix ? mix : [:])
     }
 
     /// How much of the singer is left in karaoke mode. Soft is a guide under your own voice, as quiet as settings say.
@@ -527,7 +531,7 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
         if play && !activateSession() { return }
         playWhenReady = play
         buffering = true
-        mix = startMix(item.track)
+        mix = startMix(item)
         changed()
         save()
         loadTask = Task { @MainActor [weak self] in
@@ -700,7 +704,7 @@ final class PlaybackManager: StemEngineListener, @unchecked Sendable {
         guard let item = current else { return }
         durationMs = item.track.durationMs
         waveform = nil
-        mix = startMix(item.track)
+        mix = startMix(item)
         source = currentEntry != nil ? .Stems : .Original
         stemNames = Stems.sort(currentEntry?.stems.map(\.name) ?? ["mix"])
         pushMix()