fix(scoreboard): the strip finally reaches the preview, and dev labels are orange

"Clicking on everything produces no visible effect" — the cause was upstream of the controls, and it
is the same defect as the invisible alert box (TASK 47):

- `Source.DisplaySource` returned `VideoImageSource` only for ChatBox / WebSource / MediaSource /
  AlertBox. The scoreboard's frame was rendered and published, then never displayed.
- the preview's Image has a per-type Visibility trigger for each of those and none for IsScoreboard,
  so even a correct DisplaySource would have stayed collapsed.

Both are fixed, and the layer test now pins the frame-source list AND the trigger so the pair cannot
drift apart again.

Dev-only affordances are ORANGE from now on (creator ruling): "Simulate (dev)" is a TextBlock in
Brush.Attention, so a development control can never be mistaken for product. The panel test pins the
colour.

ytLive.Tests/ScoreboardPropertiesTests.cs

@@ -25,6 +25,10 @@ public class ScoreboardPropertiesTests
         Assert.Contains("Sign in to YouTube to use the scoreboard.", block);
         Assert.Contains("CanUseScoreboard, Converter={StaticResource InverseBoolToVis}", block);
 
+        // Dev-only labels are ORANGE, so a development affordance cannot pass for product.
+        Assert.Contains("Simulate (dev)", block);
+        Assert.Contains("Brush.Attention", block);
+
         // The master switch sits in the same panel: off hides the strip and stops the polling.
         Assert.Contains("ScoreboardEnabled", panel);