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>

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);
+    }
+}

src/ClaudeDo.Ui/Views/Modals/SettingsModalView.axaml

@@ -135,7 +135,7 @@
                           <ComboBox Grid.Column="2" ItemsSource="{Binding EffortLevels}"
                                     SelectedItem="{Binding Effort, Mode=TwoWay}"
                                     HorizontalAlignment="Stretch"/>
-                          <NumericUpDown Grid.Column="4" Value="{Binding MaxTurns, Mode=TwoWay}"
+                          <NumericUpDown Grid.Column="4" Value="{Binding MaxTurns, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"
                                          Minimum="1" Maximum="200" Increment="1" FormatString="0"/>
                         </Grid>
                         <TextBlock Classes="meta" Opacity="0.6" TextWrapping="Wrap"
@@ -147,7 +147,7 @@
                 </ItemsControl>
                 <StackPanel Spacing="4" Margin="0,4,0,0">
                   <TextBlock Classes="eyebrow" Text="{loc:Tr settings.general.maxTurnsCeiling}"/>
-                  <NumericUpDown Value="{Binding General.MaxTurnsCeiling, Mode=TwoWay}"
+                  <NumericUpDown Value="{Binding General.MaxTurnsCeiling, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"
                                  Minimum="1" Maximum="200" Increment="10" FormatString="0"
                                  HorizontalAlignment="Left" Width="140"/>
                   <TextBlock Text="{loc:Tr settings.general.maxTurnsCeilingHint}"
@@ -156,7 +156,7 @@
               </StackPanel>
               <StackPanel Spacing="4">
                 <TextBlock Classes="field-label" Text="{loc:Tr settings.general.maxParallelExecutions}"/>
-                <NumericUpDown Value="{Binding General.MaxParallelExecutions, Mode=TwoWay}"
+                <NumericUpDown Value="{Binding General.MaxParallelExecutions, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"
                                Minimum="1" Maximum="20" Increment="1" FormatString="0"
                                HorizontalAlignment="Left" Width="140"/>
                 <TextBlock Text="{loc:Tr settings.general.maxParallelExecutionsHint}"
@@ -169,13 +169,13 @@
                 <StackPanel Orientation="Horizontal" Spacing="16">
                   <StackPanel Spacing="4">
                     <TextBlock Classes="eyebrow" Text="{loc:Tr settings.general.usageGateFiveHourPct}"/>
-                    <NumericUpDown Value="{Binding General.UsageGateFiveHourPct, Mode=TwoWay}"
+                    <NumericUpDown Value="{Binding General.UsageGateFiveHourPct, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"
                                    Minimum="0" Maximum="100" Increment="5" FormatString="0"
                                    HorizontalAlignment="Left" Width="140"/>
                   </StackPanel>
                   <StackPanel Spacing="4">
                     <TextBlock Classes="eyebrow" Text="{loc:Tr settings.general.usageGateSevenDayPct}"/>
-                    <NumericUpDown Value="{Binding General.UsageGateSevenDayPct, Mode=TwoWay}"
+                    <NumericUpDown Value="{Binding General.UsageGateSevenDayPct, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"
                                    Minimum="0" Maximum="100" Increment="5" FormatString="0"
                                    HorizontalAlignment="Left" Width="140"/>
                   </StackPanel>
@@ -234,7 +234,7 @@
                 <CheckBox IsChecked="{Binding Worktrees.WorktreeAutoCleanupEnabled, Mode=TwoWay}"
                           Content="{loc:Tr settings.worktrees.autoCleanup}"
                           VerticalAlignment="Center"/>
-                <NumericUpDown Value="{Binding Worktrees.WorktreeAutoCleanupDays, Mode=TwoWay}"
+                <NumericUpDown Value="{Binding Worktrees.WorktreeAutoCleanupDays, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"
                                Width="130" Minimum="1" Maximum="365" Increment="1" FormatString="0"
                                IsEnabled="{Binding Worktrees.WorktreeAutoCleanupEnabled}"/>
                 <TextBlock Classes="body" Text="{loc:Tr settings.worktrees.days}" VerticalAlignment="Center"/>
@@ -384,7 +384,7 @@
               <StackPanel Orientation="Horizontal" Spacing="8" VerticalAlignment="Center">
                 <TextBlock Classes="field-label" Text="{loc:Tr settings.prime.dailyPrepMaxTasks}" VerticalAlignment="Center"/>
                 <NumericUpDown Minimum="1" Maximum="50" Increment="1" Width="100" FormatString="0"
-                               Value="{Binding Prime.DailyPrepMaxTasks, Mode=TwoWay}"/>
+                               Value="{Binding Prime.DailyPrepMaxTasks, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"/>
               </StackPanel>
             </StackPanel>
           </ScrollViewer>
@@ -466,7 +466,7 @@
                 </StackPanel>
                 <StackPanel Spacing="4">
                   <TextBlock Classes="field-label" Text="{loc:Tr settings.onlineInbox.pollIntervalLabel}"/>
-                  <NumericUpDown Value="{Binding OnlineInbox.PollIntervalSeconds, Mode=TwoWay}"
+                  <NumericUpDown Value="{Binding OnlineInbox.PollIntervalSeconds, Mode=TwoWay, Converter={StaticResource KeepLastNumber}}"
                                  Minimum="10" Maximum="3600" Increment="10" FormatString="0"
                                  HorizontalAlignment="Left" Width="140"/>
                 </StackPanel>