feat(scoreboard): the properties are inert until the creator is signed in

The scoreboard is fed by the signed-in channel (chat events + channel stats), so offering its
switches to a signed-out user is a dead control. The whole block now binds IsEnabled to IsConnected
and shows one line — "Sign in to YouTube to use the scoreboard." — instead of looking broken.

Nothing extra is needed to re-enable: IsConnected already raises on sign-in/out, so the block
comes alive the moment the session lands.

Guard: ScoreboardPropertiesTests asserts the IsEnabled binding, the signed-out hint and that the
hint is gated by the inverse converter.

ytLive.Tests/ScoreboardPropertiesTests.cs

@@ -17,6 +17,14 @@ 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.
+        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("Sign in to YouTube to use the scoreboard.", block);
+        Assert.Contains("InverseBoolToVis", block);
+
         // The master switch sits in the same panel: off hides the strip and stops the polling.
         Assert.Contains("ScoreboardEnabled", panel);
 
@@ -39,8 +47,6 @@ public class ScoreboardPropertiesTests
 
         // Theme widgets only: the colour pickers are the app's drop-down (YtComboBox), never a
         // hand-built menu, and the toggles are YtCheckBox.
-        var block = panel[panel.IndexOf("Scoreboard config", System.StringComparison.Ordinal)..];
-        block = block[..block.IndexOf("<!-- Countdown config -->", System.StringComparison.Ordinal)];
         Assert.Contains("Style=\"{StaticResource YtComboBox}\"", block);
         Assert.Contains("Style=\"{StaticResource YtCheckBox}\"", block);
         Assert.DoesNotContain("ContextMenu", block);