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

Controls/SettingsDrawer.xaml

@@ -2,7 +2,8 @@
              xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
              xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
              xmlns:models="clr-namespace:ytLive.Models"
-             xmlns:services="clr-namespace:ytLive.Services">
+             xmlns:services="clr-namespace:ytLive.Services"
+             xmlns:notification="clr-namespace:Notification.Core;assembly=Notification.Core">
     <!-- App Settings, as a RIGHT-EDGE PULL-OUT (creator ruling 2026-11-01): the gear tab opens
          this drawer immediately, in the same animated-width pattern as Stream Settings / YPP /
          Socials / TEST. It used to be a centred modal overlay, which then needed its own scroll
@@ -25,6 +26,19 @@
                          IsReadOnly="True" Foreground="{StaticResource Brush.OnSurfaceVariant}" VerticalAlignment="Center"/>
             </DockPanel>
 
+            <TextBlock Text="Slide-out alerts" Style="{StaticResource SectionHeader}" Margin="0,6,0,4"/>
+            <TextBlock Text="Position" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Margin="10,0,0,2"/>
+            <ComboBox Style="{StaticResource YtComboBox}" Margin="10,0,0,8"
+                      SelectedValue="{Binding ToastPosition, Mode=TwoWay}" SelectedValuePath="Tag"
+                      ToolTip="Choose where notifications appear inside the app window.">
+                <ComboBoxItem Content="Top left" Tag="{x:Static notification:NotificationPosition.TopLeft}"/>
+                <ComboBoxItem Content="Top right" Tag="{x:Static notification:NotificationPosition.TopRight}"/>
+                <ComboBoxItem Content="Center left" Tag="{x:Static notification:NotificationPosition.CenterLeft}"/>
+                <ComboBoxItem Content="Center right" Tag="{x:Static notification:NotificationPosition.CenterRight}"/>
+                <ComboBoxItem Content="Bottom left" Tag="{x:Static notification:NotificationPosition.BottomLeft}"/>
+                <ComboBoxItem Content="Bottom right" Tag="{x:Static notification:NotificationPosition.BottomRight}"/>
+            </ComboBox>
+
             <!-- ═══ BRAND ═══ -->
             <TextBlock Text="Brand logo" Style="{StaticResource SectionHeader}" Margin="0,6,0,4"/>
             <TextBlock Text="Image file (shown on every scene)" Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Margin="10,0,0,2"/>