ui: stretch tab content so cards anchor to the top

TabItem defaults HorizontalContentAlignment + VerticalContentAlignment
to Center, which made the General tab's single Card float in the
vertical middle of the tab area with a wall of empty space above it.
Setting both to Stretch on the TabControl propagates to every TabItem
so each tab's ScrollViewer fills the available area and content sits
naturally at the top.

Author: carterscode

src/GamerGuardian/UI/SettingsWindow.xaml

@@ -44,7 +44,9 @@
                 </StackPanel>
             </Grid>
 
-            <TabControl x:Name="MainTabs" Padding="0,8,0,0">
+            <TabControl x:Name="MainTabs" Padding="0,8,0,0"
+                        HorizontalContentAlignment="Stretch"
+                        VerticalContentAlignment="Stretch">
 
                 <TabItem Header="General">
                     <ScrollViewer VerticalScrollBarVisibility="Auto" Padding="4,8,8,0">