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.

ViewModels/SocialsDialogViewModel.cs

@@ -42,6 +42,11 @@ public sealed class SocialsDialogViewModel : ViewModelBase
         _signInProvider = signInProvider;
         _signOutAction = signOutAction;
         _barEnabled = current?.BarEnabled ?? true;
+        _barTextColor = current?.BarTextColor ?? "#FFFFFF";
+        _barBackgroundEnabled = current?.BarBackgroundEnabled ?? false;
+        _barBackgroundColor = current?.BarBackgroundColor ?? "#000000";
+        _barBackgroundOpacity = current?.BarBackgroundOpacity ?? 0.55;
+        _barTextShadow = current?.BarTextShadow ?? false;
         _account = account;
         _isSignedIn = account != null;
 
@@ -64,6 +69,47 @@ public sealed class SocialsDialogViewModel : ViewModelBase
         set => SetProperty(ref _barEnabled, value);
     }
 
+    // ─── Bar appearance (creator request 2026-10-30) ───────────────────────────
+
+    private string _barTextColor = "#FFFFFF";
+    public string BarTextColor
+    {
+        get => _barTextColor;
+        set => SetProperty(ref _barTextColor, value);
+    }
+
+    private bool _barBackgroundEnabled;
+    public bool BarBackgroundEnabled
+    {
+        get => _barBackgroundEnabled;
+        set => SetProperty(ref _barBackgroundEnabled, value);
+    }
+
+    private string _barBackgroundColor = "#000000";
+    public string BarBackgroundColor
+    {
+        get => _barBackgroundColor;
+        set => SetProperty(ref _barBackgroundColor, value);
+    }
+
+    private double _barBackgroundOpacity = 0.55;
+    public double BarBackgroundOpacity
+    {
+        get => _barBackgroundOpacity;
+        set => SetProperty(ref _barBackgroundOpacity, value);
+    }
+
+    private bool _barTextShadow;
+    public bool BarTextShadow
+    {
+        get => _barTextShadow;
+        set => SetProperty(ref _barTextShadow, value);
+    }
+
+    /// <summary>The colours offered for the text and the plate — the same catalogue the border
+    /// picker uses, so there is one list of "the app's colours" (TASK 60/63).</summary>
+    public IReadOnlyList<NamedColor> ColorOptions => BorderColorPalette.All;
+
     public bool IsSignedIn
     {
         get => _isSignedIn;
@@ -275,7 +321,15 @@ public sealed class SocialsDialogViewModel : ViewModelBase
     {
         var entries = CommittedEntries ?? throw new InvalidOperationException("Save was not completed.");
         if (entries.Count == 0) return null;
-        var config = new SocialsConfig { BarEnabled = BarEnabled };
+        var config = new SocialsConfig
+        {
+            BarEnabled = BarEnabled,
+            BarTextColor = BarTextColor,
+            BarBackgroundEnabled = BarBackgroundEnabled,
+            BarBackgroundColor = BarBackgroundColor,
+            BarBackgroundOpacity = BarBackgroundOpacity,
+            BarTextShadow = BarTextShadow,
+        };
         foreach (var entry in entries)
             config.Entries.Add(entry);
         return config;