Name a monospace font list instead of the generic monospace

The generic name only resolves through fontconfig, so on Windows the
windows fell back to the proportional default.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.App/Dialogs/MessagesDialog.axaml

@@ -1,5 +1,6 @@
 <Window xmlns="https://github.com/avaloniaui"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:theming="using:Nonemm.App.Theming"
         x:Class="Nonemm.App.Dialogs.MessagesDialog"
         Title="Function key messages" Width="620" Height="520"
         WindowStartupLocation="CenterOwner" KeyDown="OnKeyDown">
@@ -31,7 +32,7 @@
       <ScrollViewer HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto">
         <Grid>
           <StackPanel Name="Highlight" Margin="6,4" IsHitTestVisible="False" ZIndex="1" />
-          <TextBox Name="TextArea" AcceptsReturn="True" AcceptsTab="False" FontFamily="monospace"
+          <TextBox Name="TextArea" AcceptsReturn="True" AcceptsTab="False" FontFamily="{x:Static theming:Fonts.Mono}"
                    FontSize="12" Padding="6,4" TextWrapping="NoWrap" BorderThickness="0"
                    Background="Transparent" Foreground="Transparent"
                    VerticalContentAlignment="Top" />