chore: add import Foundation and clarifying comments to context menu code

Music/ContentView.swift

@@ -366,6 +366,8 @@ struct ContentView: View {
             onAddToLastPlaylist: { track in
                 try? playlist.addTrackToLastUsedPlaylist(track)
             },
+            // Outer nil hides the "Remove from Playlist" menu item when not in a playlist view.
+            // Inner re-check defends against the playlist being deselected between menu display and action.
             onRemoveFromPlaylist: playlist.selectedPlaylist != nil ? { track in
                 if let selected = playlist.selectedPlaylist {
                     try? playlist.removeTrack(track, from: selected)

Music/Models/TrackContextMenuConfig.swift

@@ -1,8 +1,13 @@
+import Foundation
+
+// `nonisolated` opts this struct out of the project-wide `SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor`
+// setting (same reason as Track, Playlist, etc.). `Sendable` is omitted because the closure
+// properties are not Sendable — the config is created and consumed exclusively on @MainActor.
 nonisolated struct TrackContextMenuConfig {
     let playlists: [Playlist]
     let lastUsedPlaylistName: String?
     let selectedPlaylist: Playlist?
     let onAddToPlaylist: (Track, Playlist) -> Void
-    let onAddToLastPlaylist: ((Track) -> Void)?
+    let onAddToLastPlaylist: ((Track) -> Void)?  // nil hides the "Add to [last]" button; always non-nil in practice
     let onRemoveFromPlaylist: ((Track) -> Void)?
 }