fix(stream): keep recovery notifications and stop on the dispatcher

The FFmpeg ProcessFailed event arrives off-thread. Start recovery on the UI
dispatcher, publish retry status there, and route StopStream back to the
dispatcher before changing bound state or alert collections. This prevents a
recovery exception from becoming an unhandled WPF CollectionView crash.

AttentionAlertStackTests covers worker-thread recovery reporting and stop.
616 tests green, clean build with 0 warnings.

ytLive.Tests/AttentionAlertStackTests.cs

@@ -1,6 +1,7 @@
 using System;
 using System.IO;
 using System.Linq;
+using System.Threading.Tasks;
 using Microsoft.Data.Sqlite;
 using Xunit;
 using ytLive.Models;
@@ -126,6 +127,53 @@ public sealed class AttentionAlertStackTests
         });
     }
 
+    [Fact]
+    public async Task RecoveryReportAndStop_FromWorkerThread_MarshalAlertChangesToDispatcher()
+    {
+        var tempDb = Path.Combine(Path.GetTempPath(), $"ytLlive-alerts-{Guid.NewGuid():N}.db");
+        MainViewModel.LayoutPathOverride = tempDb;
+        MainViewModel? vm = null;
+        try
+        {
+            await _app.RunAsync(async () =>
+            {
+                vm = new MainViewModel();
+                var dispatcher = System.Windows.Application.Current!.Dispatcher;
+                var changedOnUi = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+                vm.AttentionAlerts.CollectionChanged += (_, _) =>
+                    changedOnUi.TrySetResult(dispatcher.CheckAccess());
+
+                await Task.Run(() => vm.ReportRecoveryStatus(
+                    "Stream dropped — reconnecting (attempt 1 of 4)", 10));
+                Assert.True(await changedOnUi.Task.WaitAsync(TimeSpan.FromSeconds(5)));
+
+                var stopped = new TaskCompletionSource<bool>(TaskCreationOptions.RunContinuationsAsynchronously);
+                vm.PropertyChanged += (_, args) =>
+                {
+                    if (args.PropertyName == nameof(vm.StreamStatus) && vm.IsOffline)
+                        stopped.TrySetResult(dispatcher.CheckAccess());
+                };
+                vm.StreamStatus = ytLive.Models.StreamStatus.Streaming;
+
+                await Task.Run(() => vm.EndStreamCommand.Execute(null));
+                Assert.True(await stopped.Task.WaitAsync(TimeSpan.FromSeconds(5)));
+                Assert.DoesNotContain(vm.AttentionAlerts, alert => alert.Id == "stream-recovery");
+            });
+        }
+        finally
+        {
+            if (vm != null)
+                await _app.RunAsync(() =>
+                {
+                    vm.Shutdown();
+                    return Task.CompletedTask;
+                });
+            MainViewModel.LayoutPathOverride = null;
+            SqliteConnection.ClearAllPools();
+            try { File.Delete(tempDb); } catch { /* best-effort cleanup */ }
+        }
+    }
+
     private void WithVm(Action<MainViewModel> body) => WithVm((vm, _) => body(vm));
 
     private void WithVm(Action<MainViewModel, string> body)