chore(ui): clear build warnings

- Guard Windows-only ServiceController/registry calls behind SupportedOSPlatform
  and OperatingSystem.IsWindows() (CA1416)
- Initialize test-only ctor fields with null! (CS8618)
- Migrate obsolete Avalonia APIs: Watermark -> PlaceholderText,
  SystemDecorations -> WindowDecorations

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -7,7 +7,7 @@
         Width="520" Height="720"
         CanResize="True"
         MinWidth="460" MinHeight="520"
-        SystemDecorations="None"
+        WindowDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">
@@ -89,7 +89,7 @@
                 <StackPanel Spacing="4">
                   <TextBlock Classes="field-label" Text="Working directory"/>
                   <Grid ColumnDefinitions="*,Auto">
-                    <TextBox Grid.Column="0" Text="{Binding WorkingDir}" Watermark="(none)" />
+                    <TextBox Grid.Column="0" Text="{Binding WorkingDir}" PlaceholderText="(none)" />
                     <Button Grid.Column="1" Content="Browse..." Margin="8,0,0,0" Click="BrowseClicked" />
                   </Grid>
                 </StackPanel>