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.Recording.cs

@@ -77,11 +77,21 @@ public partial class MainViewModel : ViewModelBase
             // CREATOR RULING 2026-09-26: Cancel means DISCARD, not "save under the
             // default name". This used to fall through and File.Move anyway, so a
             // cancelled dialog silently kept the recording — Cancel was a lie.
-            var dialog = new RenameRecordingDialog(autoStem);
-            var creatorSaved = dialog.ShowDialog() == true;
+            //
+            // APP CLOSE (2026-11-13): a modal cannot open while the window is closing — WPF throws
+            // "Cannot call ShowDialog while a Window is closing", which killed the app mid-shutdown.
+            // So closing saves under the automatic name instead: never discard footage the creator
+            // did not choose to discard, and never hold the window hostage to a dialog.
+            var creatorSaved = true;
+            string? chosenName = null;
+            if (!_shuttingDown)
+            {
+                var dialog = new RenameRecordingDialog(autoStem);
+                creatorSaved = dialog.ShowDialog() == true;
+                chosenName = dialog.ResultFileName;
+            }
 
-            var result = CompleteRecordingSave(
-                startPath, dir, autoStem, dialog.ResultFileName, creatorSaved);
+            var result = CompleteRecordingSave(startPath, dir, autoStem, chosenName, creatorSaved);
             switch (result.Outcome)
             {
                 case RecordingSaveOutcome.Saved: