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.

Services/Compositor/SocialBarRenderer.cs

@@ -33,8 +33,11 @@ public static class SocialBarRenderer
     public const int Width = 1920;
 
     /// <summary>Returns the strip frame, or null when there is nothing to render.</summary>
-    public static VideoFrame? Render(IEnumerable<SocialEntry>? entries)
+    public static VideoFrame? Render(IEnumerable<SocialEntry>? entries, SocialBarStyle? style = null)
     {
+        var look = style ?? SocialBarStyle.Default;
+        var textBrush = look.TextBrush;
+        var glyphEffect = look.TextShadow ? ShadowEffect() : null;
         var list = entries?.Where(e => e != null).ToList();
         if (list == null || list.Count == 0) return null;
 
@@ -51,7 +54,8 @@ public static class SocialBarRenderer
             item.Children.Add(new Path
             {
                 Data = Geometry.Parse(entry.LogoData),
-                Fill = Brushes.White,
+                Fill = textBrush,
+                Effect = glyphEffect,
                 Width = 24,
                 Height = 24,
                 Stretch = Stretch.Uniform,
@@ -60,7 +64,8 @@ public static class SocialBarRenderer
             item.Children.Add(new TextBlock
             {
                 Text = entry.Handle,
-                Foreground = Brushes.White,
+                Foreground = textBrush,
+                Effect = glyphEffect,
                 FontSize = 14,
                 VerticalAlignment = VerticalAlignment.Center,
                 Margin = new Thickness(8, 0, 0, 0),
@@ -77,7 +82,22 @@ public static class SocialBarRenderer
             Effect = new DropShadowEffect { Color = Color.FromRgb(0x2e, 0xcc, 0x71), BlurRadius = 18, ShadowDepth = 0, Opacity = 0.9 },
         };
         var centered = new Grid { HorizontalAlignment = HorizontalAlignment.Center, Margin = new Thickness(0, GlowPad, 0, 0) };
-        centered.Children.Add(row);
+        if (look.BackgroundEnabled)
+        {
+            // The plate hugs the entries, so it reads as a caption bar rather than a band across
+            // the frame; 20px radius matches the pill language used elsewhere.
+            centered.Children.Add(new Border
+            {
+                Background = look.BackgroundBrush,
+                CornerRadius = new CornerRadius(20),
+                Padding = new Thickness(18, 6, 18, 6),
+                Child = row,
+            });
+        }
+        else
+        {
+            centered.Children.Add(row);
+        }
         container.Children.Add(centered);
 
         container.Measure(new Size(Width, height));
@@ -102,4 +122,52 @@ public static class SocialBarRenderer
 
         return new VideoFrame(Width, height, pixels);
     }
+
+    private static DropShadowEffect ShadowEffect() => new()
+    {
+        Color = Colors.Black,
+        BlurRadius = 4,
+        ShadowDepth = 2,
+        Direction = 315,
+        Opacity = 0.85,
+    };
+}
+
+/// <summary>How the social bar is painted (creator request 2026-10-30: text colour, an optional
+/// plate with its own colour and opacity, and a text drop shadow).
+///
+/// <para>It is a separate value type, not <see cref="SocialsConfig"/>, so the renderer stays a
+/// pure function of "entries + look" and the preview/output can never read a half-applied
+/// config mid-edit.</para></summary>
+public readonly record struct SocialBarStyle(
+    string TextColor,
+    bool BackgroundEnabled,
+    string BackgroundColor,
+    double BackgroundOpacity,
+    bool TextShadow)
+{
+    public static SocialBarStyle Default => new("#FFFFFF", false, "#000000", 0.55, false);
+
+    public static SocialBarStyle From(SocialsConfig? config) => config is null
+        ? Default
+        : new(config.BarTextColor, config.BarBackgroundEnabled, config.BarBackgroundColor,
+              config.BarBackgroundOpacity, config.BarTextShadow);
+
+    /// <summary>Glyph colour. A malformed hex falls back to white rather than throwing inside a
+    /// render tick — a bad colour must never kill the output.</summary>
+    public SolidColorBrush TextBrush => Solid(TextColor, 1.0, Colors.White);
+
+    /// <summary>Plate colour carrying the configured opacity in its alpha.</summary>
+    public SolidColorBrush BackgroundBrush
+        => Solid(BackgroundColor, Math.Clamp(BackgroundOpacity, 0, 1), Colors.Black);
+
+    private static SolidColorBrush Solid(string hex, double opacity, Color fallback)
+    {
+        Color colour;
+        try { colour = (Color)ColorConverter.ConvertFromString(hex); }
+        catch (Exception) { colour = fallback; }
+        var brush = new SolidColorBrush(Color.FromArgb((byte)Math.Round(opacity * 255), colour.R, colour.G, colour.B));
+        brush.Freeze();
+        return brush;
+    }
 }