fix(ui): stop NumericUpDown from writing null into non-nullable settings

Clearing the text box to type a new value sets Value to null, which the TwoWay
binding then wrote into an int/decimal target -- InvalidCastException on the
normal way of editing eight settings fields. KeepLastNumberConverter maps that
null to BindingOperations.DoNothing so the source keeps its last value.

src/ClaudeDo.Ui/Converters/KeepLastNumberConverter.cs

@@ -0,0 +1,24 @@
+using System.Globalization;
+using Avalonia.Data;
+using Avalonia.Data.Converters;
+
+namespace ClaudeDo.Ui.Converters;
+
+/// <summary>
+/// For <c>NumericUpDown.Value</c> bound to a non-nullable numeric property. The control's Value is
+/// <c>decimal?</c> and goes null the moment the text box is empty — which is exactly what happens
+/// while the user clears a value to type a new one. Writing that null into an <c>int</c>/<c>decimal</c>
+/// target throws <see cref="InvalidCastException"/>, so swallow it and leave the source untouched
+/// until a real number arrives.
+/// </summary>
+public class KeepLastNumberConverter : IValueConverter
+{
+    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
+        => value is null ? null : System.Convert.ToDecimal(value, culture);
+
+    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
+    {
+        if (value is null) return BindingOperations.DoNothing;
+        return System.Convert.ChangeType(value, Nullable.GetUnderlyingType(targetType) ?? targetType, culture);
+    }
+}