feat(scoreboard): the layer is available to ALL scenes (YT_ACTIVITY_TICKER)

Creator ruling: the Scoreboard layer exists in every scene rather than being added five times.
ScoreboardHealer.Ensure keeps exactly one per scene — healed on load like the Background and Brand
logo — so a scene can never end up with two, and 'off' is the ordinary per-scene eye toggle rather
than a delete.

It is appended as the TOPMOST layer on purpose: a full-width strip draws over the scene, and
inserting it above the background would break the Brand logo's documented invariant (directly above
the background). That choice is also why the layer-reorder test still passes untouched.

Also: a layer-list icon for the type, so the row is not blank.

Guard: ScoreboardHealerTests — one per scene, topmost, exact strip geometry, idempotent, and a
duplicate healed back to one.

ViewModels/MainViewModel.Background.cs

@@ -439,6 +439,7 @@ public partial class MainViewModel
         }
         SeedOnAirImage();
         EnsureBrandLogos();
+        EnsureScoreboards();
         OnPropertyChanged(nameof(ShowSourcesEmptyHint));
         OnPropertyChanged(nameof(LayerCount));
     }

ViewModels/MainViewModel.Scoreboard.cs

@@ -45,6 +45,18 @@ public partial class MainViewModel
         RefreshScoreboardPreviews();
     }
 
+    /// <summary>One Scoreboard layer in every scene, healed on load — "available to all scenes"
+    /// (creator ruling 2026-10-30). Per-scene visibility is the ordinary eye toggle.</summary>
+    internal void EnsureScoreboards()
+    {
+        if (ScoreboardHealer.Ensure(Scenes))
+        {
+            foreach (var scene in Scenes) _sceneGraph.InvalidateBake(scene);
+            RefreshScoreboardPreviews();
+            OnPropertyChanged(nameof(LayerCount));
+        }
+    }
+
     private void InitScoreboard()
     {
         _scoreboardPresenter = new ScoreboardPresenter(_scoreboard, _scoreboardStyle)