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

MainWindow.xaml

@@ -170,10 +170,10 @@
         <!-- ═══ OVERLAY: Settings / Bug / Feature / About ═══ -->
         <controls:OverlayHost x:Name="AboutOverlay" Grid.RowSpan="3"/>
 
-        <!-- ═══ TOASTS: non-blocking notifications (bottom-right, above footer) ═══ -->
+        <!-- ═══ TOASTS: non-blocking notifications (user-positioned, bottom-right by default) ═══ -->
         <notifications:NotificationArea x:Name="ToastArea"
                                         Grid.RowSpan="4"
-                                        Position="BottomRight" MaxItems="4"
-                                        Margin="0,0,12,96"/>
+                                        Position="{Binding ToastPosition}" MaxItems="4"
+                                        Margin="{Binding ToastAreaMargin}"/>
     </Grid>
 </Window>