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.cs

@@ -343,6 +343,7 @@ public partial class MainViewModel : ViewModelBase
 
         _layoutStore = new LayoutStore(LayoutPathOverride ?? DefaultLayoutPath);
         _activeLayoutPath = _layoutStore.ActivePath;
+        _toastPosition = _layoutStore.LoadNotificationPosition();
         _recordFolder = _layoutStore.LoadRecordFolder();
 
         // Load saved hotkey bindings (filled with defaults for any unbound actions).