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/LayoutStore.Load.cs

@@ -50,7 +50,11 @@ public partial class LayoutStore : IDisposable
 
         using (var cmd = _connection.CreateCommand())
         {
-            cmd.CommandText = "SELECT Id, BarPosition, BarEnabled FROM Socials LIMIT 1;";
+            cmd.CommandText = """
+                SELECT Id, BarPosition, BarEnabled, TextColor, BackgroundEnabled,
+                       BackgroundColor, BackgroundOpacity, TextShadow
+                FROM Socials LIMIT 1;
+                """;
             using var reader = cmd.ExecuteReader();
             if (reader.Read())
             {
@@ -58,6 +62,11 @@ public partial class LayoutStore : IDisposable
                 {
                     BarPosition = Enum.TryParse<SocialBarPosition>(reader.GetString(1), out var pos) ? pos : SocialBarPosition.Bottom,
                     BarEnabled = reader.FieldCount > 2 && !reader.IsDBNull(2) && reader.GetInt32(2) != 0,
+                    BarTextColor = reader.FieldCount > 3 && !reader.IsDBNull(3) ? reader.GetString(3) : "#FFFFFF",
+                    BarBackgroundEnabled = reader.FieldCount > 4 && !reader.IsDBNull(4) && reader.GetInt32(4) != 0,
+                    BarBackgroundColor = reader.FieldCount > 5 && !reader.IsDBNull(5) ? reader.GetString(5) : "#000000",
+                    BarBackgroundOpacity = reader.FieldCount > 6 && !reader.IsDBNull(6) ? reader.GetDouble(6) : 0.55,
+                    BarTextShadow = reader.FieldCount > 7 && !reader.IsDBNull(7) && reader.GetInt32(7) != 0,
                 };
                 var socialsId = reader.GetString(0);