feat(scoreboard): dev mode can configure the board without signing in

Creator (2026-10-30): in dev mode the "Show the scoreboard" option must always be available.

The block's gate becomes CanUseScoreboard = IsConnected || IsDevelopmentBuild. Signed-out Release
still shows the inert block with its "sign in" line; a DEBUG build is fully usable, so the strip can
be toggled and previewed before there is an account. IsConnected raises it, so signing in flips it
live.

Guard: ScoreboardPropertiesTests now asserts the CanUseScoreboard gate (enabled AND hint), and the
VM surface list includes it.

ytLive.Tests/ScoreboardPropertiesTests.cs

@@ -17,13 +17,13 @@ public class ScoreboardPropertiesTests
         // Gated on the selected layer being a scoreboard.
         Assert.Contains("SelectedElement.IsScoreboard", panel);
 
-        // Signed out: the whole block is inert (the board is fed by the signed-in channel), with a
-        // one-line explanation. Signing in re-enables it through the ordinary IsConnected raise.
+        // Signed out: the block is inert with a one-line explanation — EXCEPT in a DEBUG build, where
+        // CanUseScoreboard is true so the strip can be previewed without an account.
         var block = panel[panel.IndexOf("Scoreboard config", System.StringComparison.Ordinal)..];
         block = block[..block.IndexOf("<!-- Countdown config -->", System.StringComparison.Ordinal)];
-        Assert.Contains("IsEnabled=\"{Binding IsConnected}\"", block);
+        Assert.Contains("IsEnabled=\"{Binding CanUseScoreboard}\"", block);
         Assert.Contains("Sign in to YouTube to use the scoreboard.", block);
-        Assert.Contains("InverseBoolToVis", block);
+        Assert.Contains("CanUseScoreboard, Converter={StaticResource InverseBoolToVis}", block);
 
         // The master switch sits in the same panel: off hides the strip and stops the polling.
         Assert.Contains("ScoreboardEnabled", panel);
@@ -57,7 +57,7 @@ public class ScoreboardPropertiesTests
                  {
                      "ScoreboardTextColor", "ScoreboardBackgroundColor", "ScoreboardBackgroundOpacity",
                      "ScoreboardFontSize", "ScoreboardColorOptions", "ScoreboardShowSubscribers",
-                     "ScoreboardEnabled",
+                     "ScoreboardEnabled", "CanUseScoreboard",
                  })
             Assert.NotNull(type.GetProperty(name));
     }