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.

Models/Socials.cs

@@ -70,6 +70,50 @@ public sealed class SocialsConfig : INotifyPropertyChanged
         set => Set(ref _barPosition, value);
     }
 
+    // ─── Bar appearance (creator request 2026-10-30) ───────────────────────────
+    // The bar used to be hard-wired white-on-transparent. These five knobs are the whole
+    // appearance surface: text colour, an optional plate behind it, and a text shadow.
+
+    private string _barTextColor = "#FFFFFF";
+    /// <summary>Text and logo colour of the bar (hex, straight alpha).</summary>
+    public string BarTextColor
+    {
+        get => _barTextColor;
+        set => Set(ref _barTextColor, string.IsNullOrWhiteSpace(value) ? "#FFFFFF" : value);
+    }
+
+    private bool _barBackgroundEnabled;
+    /// <summary>Draw a rounded plate behind the entries. Off by default — the bar shipped
+    /// transparent and most creators keep it that way.</summary>
+    public bool BarBackgroundEnabled
+    {
+        get => _barBackgroundEnabled;
+        set => Set(ref _barBackgroundEnabled, value);
+    }
+
+    private string _barBackgroundColor = "#000000";
+    public string BarBackgroundColor
+    {
+        get => _barBackgroundColor;
+        set => Set(ref _barBackgroundColor, string.IsNullOrWhiteSpace(value) ? "#000000" : value);
+    }
+
+    private double _barBackgroundOpacity = 0.55;
+    /// <summary>0 = invisible plate, 1 = solid. Only meaningful when the plate is on.</summary>
+    public double BarBackgroundOpacity
+    {
+        get => _barBackgroundOpacity;
+        set => Set(ref _barBackgroundOpacity, Math.Clamp(value, 0, 1));
+    }
+
+    private bool _barTextShadow;
+    /// <summary>A soft black drop shadow behind the glyphs, for a bright or busy scene.</summary>
+    public bool BarTextShadow
+    {
+        get => _barTextShadow;
+        set => Set(ref _barTextShadow, value);
+    }
+
     public event PropertyChangedEventHandler? PropertyChanged;
     private bool Set<T>(ref T field, T value, [CallerMemberName] string? name = null)
     {