fix(lifecycle): no dialogs while the app is closing

1ce6b9c made closing stop the session first, which was right and incomplete: the stop path opens TWO
modals, and WPF forbids that during a close. The creator's #2 test produced it immediately —

  System.InvalidOperationException: Cannot set Visibility to Visible or call Show, ShowDialog, Close,
  or WindowInteropHelper.EnsureHandle while a Window is closing.

Two prompts are suppressed by a `_shuttingDown` flag, each taking its safe default rather than a prompt
nobody can answer:
- The rename dialog: the recording is saved under its automatic name. Never discard footage the creator
  did not choose to discard.
- The private-broadcast keep/delete prompt: the broadcast is KEPT (stays private on YouTube) instead of
  being deleted by an answer that was never given.

The general rule this establishes: a teardown path may not ask questions. Any future prompt reached from
stop must check IsShuttingDown.

Not covered by a unit test — the trigger is a real WPF close, not reachable from the view model. Verified
against the exception in %APPDATA%\ytLlive\startup.log (09:40:27) and it needs the creator's retest.

ViewModels/MainViewModel.Streaming.Operations.cs

@@ -690,7 +690,11 @@ public partial class MainViewModel : ViewModelBase
     {
         var title = BroadcastForm.Title;
         var (outcome, error) = await ClosePrivateBroadcastAsync(_youtubeStream, broadcastId,
-            () => KeepBroadcastPromptOverride?.Invoke(title) ?? new KeepBroadcastDialog(title).ShowDialog() == true);
+            // Closing the app cannot open a modal (WPF forbids ShowDialog while a window closes), so the
+            // safe default is KEEP: the broadcast stays on YouTube as a private video for the creator to
+            // deal with, instead of being deleted by a prompt nobody could answer.
+            () => _shuttingDown
+                || (KeepBroadcastPromptOverride?.Invoke(title) ?? new KeepBroadcastDialog(title).ShowDialog() == true));
         switch (outcome)
         {
             case PrivateBroadcastOutcome.Kept: