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.App/App.axaml

@@ -22,6 +22,7 @@
             <converters:BoolToItalicConverter      x:Key="BoolToItalic"/>
             <converters:BoolToDraftOpacityConverter x:Key="BoolToDraftOpacity"/>
             <converters:LogKindForegroundConverter  x:Key="LogKindForeground"/>
+            <converters:KeepLastNumberConverter     x:Key="KeepLastNumber"/>
 
         </ResourceDictionary>
     </Application.Resources>