fix(ui): prevent async void races and leak-on-exit

- task detail vm: LoadAsync now uses a per-call CancellationTokenSource
  so rapid TaskUpdated events can't race on _taskId / Subtasks / Tags;
  old subtask PropertyChanged handlers are torn down before Clear
- task detail vm: async void event handlers (OnTaskUpdated,
  OnWorktreeUpdated, OnSubtaskPropertyChanged) wrap work in try/catch
  so thrown exceptions can't crash the Avalonia sync context
- task detail vm: Clear cancels/disposes the load CTS so a late-arriving
  LoadAsync can't resurrect detail state after deselect
- app: DisposeAsync the ServiceProvider in a finally after the classic
  desktop lifetime ends, so WorkerClient.DisposeAsync runs and the
  SignalR connection closes cleanly instead of being abandoned

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/ClaudeDo.App/Program.cs

@@ -22,9 +22,20 @@ sealed class Program
         var factory = services.GetRequiredService<SqliteConnectionFactory>();
         SchemaInitializer.Apply(factory);
 
+        try
+        {
             BuildAvaloniaApp()
                 .StartWithClassicDesktopLifetime(args);
         }
+        finally
+        {
+            // Dispose the container so WorkerClient.DisposeAsync runs —
+            // cancels the retry loop and closes the SignalR connection cleanly
+            // instead of abandoning it.
+            try { services.DisposeAsync().AsTask().GetAwaiter().GetResult(); }
+            catch { /* best effort on shutdown */ }
+        }
+    }
 
     public static AppBuilder BuildAvaloniaApp()
         => AppBuilder.Configure<App>()

src/ClaudeDo.Ui/ViewModels/TaskDetailViewModel.cs

@@ -55,6 +55,10 @@ public partial class TaskDetailViewModel : ViewModelBase
     private string? _taskId;
     private string? _listId;
     private bool _isLoading;
+    // Cancels an in-flight LoadAsync when a new TaskUpdated event arrives
+    // before the previous load finished — prevents torn state on _taskId,
+    // Subtasks, Tags, etc.
+    private CancellationTokenSource? _loadCts;
 
     public event Action<string>? TaskChanged;
 
@@ -79,16 +83,29 @@ public partial class TaskDetailViewModel : ViewModelBase
 
     public async Task LoadAsync(string taskId)
     {
+        // Cancel any in-flight load so rapid TaskUpdated events don't race
+        // on _taskId / Subtasks / Tags. The newest caller wins.
+        var oldCts = _loadCts;
+        var cts = new CancellationTokenSource();
+        _loadCts = cts;
+        oldCts?.Cancel();
+        oldCts?.Dispose();
+        var ct = cts.Token;
+
         _taskId = taskId;
         LiveText = "";
         _formatter = new StreamLineFormatter();
 
-        var task = await _taskRepo.GetByIdAsync(taskId);
+        try
+        {
+            var task = await _taskRepo.GetByIdAsync(taskId, ct);
             if (task is null) return;
+            ct.ThrowIfCancellationRequested();
 
             if (AvailableAgents.Count == 0)
             {
                 var agents = await _worker.GetAgentsAsync();
+                ct.ThrowIfCancellationRequested();
                 AvailableAgents.AddRange(agents);
                 OnPropertyChanged(nameof(AvailableAgents));
             }
@@ -106,7 +123,7 @@ public partial class TaskDetailViewModel : ViewModelBase
                     && File.Exists(task.LogPath))
                 {
                     _formatter = new StreamLineFormatter();
-                LiveText = await Task.Run(() => _formatter.FormatFile(task.LogPath));
+                    LiveText = await Task.Run(() => _formatter.FormatFile(task.LogPath), ct);
                 }
                 StatusText = task.Status.ToString().ToLowerInvariant();
                 StatusChoice = task.Status.ToString();
@@ -132,12 +149,14 @@ public partial class TaskDetailViewModel : ViewModelBase
                 }
 
                 Tags.Clear();
-            var tags = await _taskRepo.GetTagsAsync(taskId);
+                var tags = await _taskRepo.GetTagsAsync(taskId, ct);
                 foreach (var tag in tags)
                     Tags.Add(tag);
 
+                // Tear down old subtask subscriptions before replacing them.
+                foreach (var old in Subtasks) old.PropertyChanged -= OnSubtaskPropertyChanged;
                 Subtasks.Clear();
-            var subtasks = await _subtaskRepo.GetByTaskIdAsync(taskId);
+                var subtasks = await _subtaskRepo.GetByTaskIdAsync(taskId, ct);
                 foreach (var s in subtasks)
                 {
                     var vm = SubtaskItemViewModel.From(s);
@@ -152,6 +171,11 @@ public partial class TaskDetailViewModel : ViewModelBase
 
             await LoadWorktreeAsync(taskId);
         }
+        catch (OperationCanceledException)
+        {
+            // Superseded by a newer LoadAsync — nothing to do.
+        }
+    }
 
     public async Task SaveAsync()
     {
@@ -236,6 +260,8 @@ public partial class TaskDetailViewModel : ViewModelBase
     {
         if (_isLoading || sender is not SubtaskItemViewModel vm || string.IsNullOrEmpty(vm.Id)) return;
         if (e.PropertyName is not (nameof(SubtaskItemViewModel.Title) or nameof(SubtaskItemViewModel.Completed))) return;
+        try
+        {
             await _subtaskRepo.UpdateAsync(new SubtaskEntity
             {
                 Id = vm.Id,
@@ -246,6 +272,12 @@ public partial class TaskDetailViewModel : ViewModelBase
                 CreatedAt = DateTime.UtcNow,
             });
         }
+        catch (Exception ex)
+        {
+            // async void must never throw — surface via Debug.
+            Debug.WriteLine($"[TaskDetailViewModel] Subtask update failed for {vm.Id}: {ex}");
+        }
+    }
 
     public void SetAgentFromPath(string path)
     {
@@ -261,6 +293,11 @@ public partial class TaskDetailViewModel : ViewModelBase
 
     public void Clear()
     {
+        // Cancel any load in flight so it doesn't resurrect state after Clear.
+        _loadCts?.Cancel();
+        _loadCts?.Dispose();
+        _loadCts = null;
+
         _taskId = null;
         _listId = null;
         Title = "";
@@ -408,12 +445,28 @@ public partial class TaskDetailViewModel : ViewModelBase
     private async void OnWorktreeUpdated(string taskId)
     {
         if (taskId != _taskId) return;
+        try
+        {
             await LoadWorktreeAsync(taskId);
         }
+        catch (Exception ex)
+        {
+            // async void must never throw.
+            Debug.WriteLine($"[TaskDetailViewModel] OnWorktreeUpdated failed for {taskId}: {ex}");
+        }
+    }
 
     private async void OnTaskUpdated(string taskId)
     {
         if (taskId != _taskId) return;
+        try
+        {
             await LoadAsync(taskId);
         }
+        catch (Exception ex)
+        {
+            // async void must never throw.
+            Debug.WriteLine($"[TaskDetailViewModel] OnTaskUpdated failed for {taskId}: {ex}");
+        }
+    }
 }