feat(scoreboard): a master on/off switch that also stops the polling

Creator: "even if it's not visible, we want to avoid polling."

ScoreboardEnabled is the layer's master switch in the properties panel. Off means two things:
- the strip frame is null in every scene (per-scene eye toggles stay independent), and
- the cadence policy returns None at every phase — a scoreboard nobody can see must not spend quota.

The switch is persisted (Scoreboard.Enabled), because one that resets would quietly resume polling.

The polling gate is ScoreboardPollsEnabled: master switch AND at least one scene with a visible
scoreboard — so hiding it in all five scenes also stops the polls.

Guards: ScoreboardCadenceTests adds the switch off across live/test/recording; ScoreboardProperties
pins the checkbox and the VM surface.

Services/LayoutStore.Settings.cs

@@ -51,6 +51,12 @@ public partial class LayoutStore : IDisposable
 
     public void SaveMonoOutput(bool enabled) => UpsertSetting("Audio.MonoOutput", enabled ? "1" : "0");
 
+    /// <summary>Scoreboard master switch (creator 2026-10-30). Persisted because "off" also means
+    /// "stop polling" — a switch that resets would quietly resume spending quota.</summary>
+    public bool LoadScoreboardEnabled() => GetSetting("Scoreboard.Enabled") != "0";
+
+    public void SaveScoreboardEnabled(bool enabled) => UpsertSetting("Scoreboard.Enabled", enabled ? "1" : "0");
+
     /// <summary>The user's chosen record output folder (TASK 18), or null to use the
     /// default <c>%APPDATA%\ytLlive\recordings</c>. Restored at startup.</summary>
     public string? LoadRecordFolder()