feat(settings): position slide-out notifications

App Settings now chooses one of six ToastArea positions (left/right ×
top/center/bottom), defaulting to the existing bottom-right. The setting is
persisted per profile and the toast area moves immediately; edge gutters keep
bottom notifications above the footer.

ToastPositionIntegrationTests covers live repositioning and restart persistence.
617 tests green, clean build with 0 warnings.

Reference scan: OBS Notifier supports user-configured notification placement:
https://github.com/DmitriySalnikov/OBSNotifier

ViewModels/MainViewModel.Settings.cs

@@ -1,3 +1,6 @@
+using System.Windows;
+using Notification.Core;
+
 namespace ytLive.ViewModels;
 
 /// <summary>
@@ -10,6 +13,33 @@ namespace ytLive.ViewModels;
 public partial class MainViewModel
 {
     private bool _settingsDrawerOpen;
+    private NotificationPosition _toastPosition = NotificationPosition.BottomRight;
+
+    public NotificationPosition ToastPosition
+    {
+        get => _toastPosition;
+        set
+        {
+            if (!IsSupportedToastPosition(value) || !SetProperty(ref _toastPosition, value)) return;
+            _layoutStore.SaveNotificationPosition(value);
+            OnPropertyChanged(nameof(ToastAreaMargin));
+        }
+    }
+
+    public Thickness ToastAreaMargin => ToastPosition switch
+    {
+        NotificationPosition.TopLeft => new Thickness(12, 12, 0, 0),
+        NotificationPosition.TopRight => new Thickness(0, 12, 12, 0),
+        NotificationPosition.CenterLeft => new Thickness(12, 0, 0, 0),
+        NotificationPosition.CenterRight => new Thickness(0, 0, 12, 0),
+        NotificationPosition.BottomLeft => new Thickness(12, 0, 0, 96),
+        _ => new Thickness(0, 0, 12, 96),
+    };
+
+    private static bool IsSupportedToastPosition(NotificationPosition position)
+        => position is NotificationPosition.TopLeft or NotificationPosition.TopRight
+            or NotificationPosition.CenterLeft or NotificationPosition.CenterRight
+            or NotificationPosition.BottomLeft or NotificationPosition.BottomRight;
 
     public bool SettingsDrawerOpen
     {