Fix SwiftData persistence concurrency stability

apps/apple/VelodyMac/Sources/MacLibraryView.swift

@@ -14,7 +14,7 @@ struct MacLibraryView: View {
             Text("Backend Connection")
                 .font(.title2)
 
-            TextField("http://localhost:3000", text: $viewModel.serverURLString)
+            TextField("http://localhost:3007", text: $viewModel.serverURLString)
                 .textFieldStyle(.roundedBorder)
                 .onSubmit {
                     viewModel.persistServerURLSelection()

apps/apple/VelodyMac/Sources/MacLibraryViewModel.swift

@@ -16,6 +16,7 @@ final class MacLibraryViewModel {
     var discoveredTrackCount = 0
     var isScanning = false
     var nowPlayingState = NowPlayingState()
+    var playbackErrorMessage: String?
 
     var serverURLString: String
     var deviceRegistrationStatus = "Not registered."
@@ -70,9 +71,11 @@ final class MacLibraryViewModel {
         self.serverURLString = userDefaults.string(forKey: Self.serverURLDefaultsKey)
             ?? ServerEnvironment.defaultLocalBaseURL.absoluteString
         self.nowPlayingState = playbackController.nowPlayingState
+        self.playbackErrorMessage = Self.playbackErrorMessage(from: playbackController.nowPlayingState.error)
 
         playbackController.onStateChange = { [weak self] state in
             self?.nowPlayingState = state
+            self?.playbackErrorMessage = Self.playbackErrorMessage(from: state.error)
         }
 
         if let url = folderAccessService.storedFolderURL() {
@@ -215,10 +218,6 @@ final class MacLibraryViewModel {
         }
     }
 
-    var playbackErrorMessage: String? {
-        nowPlayingState.error?.errorDescription
-    }
-
     func uploadSelectedTrack() async {
         guard let selectedTrackID else {
             lastUploadStatus = "Select a local track before uploading."
@@ -789,6 +788,10 @@ final class MacLibraryViewModel {
     private static let deviceIdKey = "velody.device-id"
     private static let bootstrapTokenKey = "velody.bootstrap-token"
     private static let playbackSessionDefaultsKey = "velody.playback.session"
+
+    private static func playbackErrorMessage(from error: PlaybackError?) -> String? {
+        error?.errorDescription ?? error?.localizedDescription
+    }
 }
 
 private enum UploadOutcome {
@@ -803,7 +806,7 @@ private enum BackendConnectionError: LocalizedError {
     var errorDescription: String? {
         switch self {
         case .invalidServerURL:
-            return "Enter a valid backend URL, such as http://localhost:3000."
+            return "Enter a valid backend URL, such as http://localhost:3007."
         case .missingDeviceIdentity:
             return "Register this Mac before uploading or sending a heartbeat."
         }