feat(socials): bar appearance — text colour, background plate, opacity, text shadow

In the Socials drawer, under the show/hide switch: text colour, an optional background plate
with its own colour and opacity, and a text drop shadow. The bar had been hard-wired
white-on-transparent since it shipped.

- `SocialsConfig` gains five additive properties (BarTextColor, BarBackgroundEnabled,
  BarBackgroundColor, BarBackgroundOpacity, BarTextShadow).
- Persistence: five new Socials columns, added by the per-column idempotent migration pattern
  (a half-migrated DB heals instead of failing), written in Save and read back in Load with
  defaults for pre-existing rows.
- `SocialBarRenderer.Render(entries, SocialBarStyle?)` — the look is a separate value type so the
  renderer stays a pure "entries + look" function and the preview can never read a half-applied
  config mid-edit. A malformed hex falls back to white/black instead of throwing inside a render
  tick.
- The plate hugs the entries (rounded 20px, padding) so it reads as a caption bar, not a band
  across the frame. Off by default, because transparent is what creators have today.
- The colour pickers reuse `BorderColorPalette` — one catalogue of the app's colours, per the
  TASK 60/63 rule.

Guard: SocialBarAppearanceTests paints the real strip and inspects pixels — default is white with
no plate, a red text colour reaches the glyphs, a 55% blue plate yields half-transparent blue
pixels, and the shadow adds dark pixels that were not there before.

SocialsDialog.xaml

@@ -20,10 +20,49 @@
         <TextBlock Grid.Row="0" Text="Social Media Site Promotion" FontSize="18" FontWeight="Bold"
                    Foreground="{StaticResource Brush.OnSurface}" Margin="0,0,0,12"/>
 
-        <!-- Show/hide the bar on the stream -->
-        <CheckBox Grid.Row="1" IsChecked="{Binding BarEnabled}" Style="{StaticResource YtCheckBox}" Margin="0,0,0,10">
-            <TextBlock Text="Show the social bar on the stream" Foreground="{StaticResource Brush.OnSurface}" FontSize="13"/>
-        </CheckBox>
+        <!-- Show/hide the bar on the stream + how it is painted -->
+        <StackPanel Grid.Row="1" Margin="0,0,0,10">
+            <CheckBox IsChecked="{Binding BarEnabled}" Style="{StaticResource YtCheckBox}">
+                <TextBlock Text="Show the social bar on the stream" Foreground="{StaticResource Brush.OnSurface}" FontSize="13"/>
+            </CheckBox>
+
+            <StackPanel Margin="18,6,0,0" IsEnabled="{Binding BarEnabled}">
+                <DockPanel Margin="0,2,0,0">
+                    <TextBlock DockPanel.Dock="Left" Text="Text colour" VerticalAlignment="Center"
+                               Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Width="96"/>
+                    <ComboBox Style="{StaticResource YtComboBox}" Height="24" FontSize="11"
+                              ItemsSource="{Binding ColorOptions}" DisplayMemberPath="Name"
+                              SelectedValuePath="Hex" SelectedValue="{Binding BarTextColor, Mode=TwoWay}"/>
+                </DockPanel>
+
+                <CheckBox IsChecked="{Binding BarBackgroundEnabled}" Style="{StaticResource YtCheckBox}" Margin="0,8,0,0">
+                    <TextBlock Text="Background plate" Foreground="{StaticResource Brush.OnSurface}" FontSize="12"/>
+                </CheckBox>
+
+                <StackPanel Margin="18,4,0,0" IsEnabled="{Binding BarBackgroundEnabled}">
+                    <DockPanel>
+                        <TextBlock DockPanel.Dock="Left" Text="Plate colour" VerticalAlignment="Center"
+                                   Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Width="96"/>
+                        <ComboBox Style="{StaticResource YtComboBox}" Height="24" FontSize="11"
+                                  ItemsSource="{Binding ColorOptions}" DisplayMemberPath="Name"
+                                  SelectedValuePath="Hex" SelectedValue="{Binding BarBackgroundColor, Mode=TwoWay}"/>
+                    </DockPanel>
+                    <DockPanel Margin="0,6,0,0">
+                        <TextBlock DockPanel.Dock="Left" Text="Opacity" VerticalAlignment="Center"
+                                   Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11" Width="96"/>
+                        <TextBlock DockPanel.Dock="Right" VerticalAlignment="Center" Width="42" TextAlignment="Right"
+                                   Foreground="{StaticResource Brush.OnSurfaceVariant}" FontSize="11"
+                                   Text="{Binding BarBackgroundOpacity, StringFormat={}{0:P0}}"/>
+                        <Slider Minimum="0" Maximum="1" SmallChange="0.05" LargeChange="0.1"
+                                Value="{Binding BarBackgroundOpacity, Mode=TwoWay}" VerticalAlignment="Center"/>
+                    </DockPanel>
+                </StackPanel>
+
+                <CheckBox IsChecked="{Binding BarTextShadow}" Style="{StaticResource YtCheckBox}" Margin="0,8,0,0">
+                    <TextBlock Text="Text drop shadow" Foreground="{StaticResource Brush.OnSurface}" FontSize="12"/>
+                </CheckBox>
+            </StackPanel>
+        </StackPanel>
 
         <!-- Sign-in gate: the first slot is always your YouTube channel -->
         <Border Grid.Row="2" Background="{StaticResource Brush.SurfaceContainer}" CornerRadius="5" Padding="10,8" Margin="0,0,0,10"