Fix styles as Styles (not ResourceDictionary); replace DataTriggers with width converters; add IsZeroConverter; remove unsupported properties; fix ancestor command bindings; bind SplitView props directly.

src/AdvancedCalculator/Converters/WidthToSplitViewModeConverter.cs

@@ -0,0 +1,30 @@
+using System;
+using System.Globalization;
+using Avalonia.Controls;
+using Avalonia.Data.Converters;
+
+namespace AdvancedCalculator.Converters;
+
+public class WidthToSplitViewModeConverter : IValueConverter
+{
+    public static readonly WidthToSplitViewModeConverter Instance = new();
+
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo? culture)
+    {
+        if (value is double width)
+        {
+            double threshold = 640;
+            if (parameter is double p)
+                threshold = p;
+            else if (parameter is string s && double.TryParse(s, NumberStyles.Number, CultureInfo.InvariantCulture, out var parsed))
+                threshold = parsed;
+
+            return width < threshold ? SplitViewDisplayMode.Overlay : SplitViewDisplayMode.Inline;
+        }
+        return SplitViewDisplayMode.Inline;
+    }
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo? culture)
+        => throw new NotSupportedException();
+}
+