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/MainViewModel.Socials.cs

@@ -111,7 +111,7 @@ public partial class MainViewModel
     private void RenderSocialBarFrame()
     {
         _socialBarFrame = _socials != null && _socials.BarEnabled
-            ? SocialBarRenderer.Render(_socials.Entries)
+            ? SocialBarRenderer.Render(_socials.Entries, SocialBarStyle.From(_socials))
             : null;
     }