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.Save.cs

@@ -314,11 +314,20 @@ public partial class LayoutStore : IDisposable
             var socialsId = Guid.NewGuid().ToString();
             using (var cmd = _connection.CreateCommand())
             {
-                cmd.CommandText = "INSERT INTO Socials (Id, BarPosition, BarEnabled) VALUES ($id, $pos, $enabled);";
+                cmd.CommandText = """
+                    INSERT INTO Socials (Id, BarPosition, BarEnabled, TextColor, BackgroundEnabled,
+                                         BackgroundColor, BackgroundOpacity, TextShadow)
+                    VALUES ($id, $pos, $enabled, $text, $bgen, $bg, $bgopacity, $shadow);
+                    """;
                 cmd.Transaction = tx;
                 cmd.Parameters.AddWithValue("$id", socialsId);
                 cmd.Parameters.AddWithValue("$pos", socials.BarPosition.ToString());
                 cmd.Parameters.AddWithValue("$enabled", socials.BarEnabled ? 1 : 0);
+                cmd.Parameters.AddWithValue("$text", socials.BarTextColor);
+                cmd.Parameters.AddWithValue("$bgen", socials.BarBackgroundEnabled ? 1 : 0);
+                cmd.Parameters.AddWithValue("$bg", socials.BarBackgroundColor);
+                cmd.Parameters.AddWithValue("$bgopacity", socials.BarBackgroundOpacity);
+                cmd.Parameters.AddWithValue("$shadow", socials.BarTextShadow ? 1 : 0);
                 cmd.ExecuteNonQuery();
             }