feat(mission-control): column picker for the overview grid

Auto keeps the pane-count heuristic; 1-4 fixes the column count (capped at
the pane count). Persisted to ui.config.json as MissionControlColumns.

src/ClaudeDo.Ui/Converters/SimpleConverters.cs

@@ -19,6 +19,14 @@ public sealed class HexBrushConverter : OneWayConverter
             : Avalonia.Application.Current?.TryGetResource(parameter as string ?? "", null, out var res) == true ? res : null;
 }
 
+/// <summary>True when the value's string form equals the parameter (segmented-switch active state).</summary>
+public sealed class EqualsParameterConverter : OneWayConverter
+{
+    public static EqualsParameterConverter Instance { get; } = new();
+    public override object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => Equals(value?.ToString(), parameter as string);
+}
+
 public sealed class UpperCaseConverter : OneWayConverter
 {
     public static UpperCaseConverter Instance { get; } = new();