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

Services/LayoutStore.Settings.cs

@@ -1,4 +1,5 @@
 using Microsoft.Data.Sqlite;
+using Notification.Core;
 using ytLive.Services.Compositor;
 using ytLive.Models;
 
@@ -6,6 +7,25 @@ namespace ytLive.Services;
 
 public partial class LayoutStore : IDisposable
 {
+    public NotificationPosition LoadNotificationPosition()
+    {
+        var value = GetSetting("Notifications.Position");
+        return Enum.TryParse(value, true, out NotificationPosition position) && IsSupportedNotificationPosition(position)
+            ? position
+            : NotificationPosition.BottomRight;
+    }
+
+    public void SaveNotificationPosition(NotificationPosition position)
+    {
+        if (!IsSupportedNotificationPosition(position)) position = NotificationPosition.BottomRight;
+        UpsertSetting("Notifications.Position", position.ToString());
+    }
+
+    private static bool IsSupportedNotificationPosition(NotificationPosition position)
+        => position is NotificationPosition.TopLeft or NotificationPosition.TopRight
+            or NotificationPosition.CenterLeft or NotificationPosition.CenterRight
+            or NotificationPosition.BottomLeft or NotificationPosition.BottomRight;
+
     /// <summary>The last picked mic source name, or null if never chosen. Restored
     /// at startup so the sound bar reconnects to the same, already-vetted device.</summary>
     public string? LoadMicSourceName()