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/Islands/TasksIslandView.axaml

@@ -51,7 +51,7 @@
                     Foreground="{DynamicResource TextFaintBrush}"/>
         </Border>
         <TextBox Grid.Column="1" x:Name="AddTaskBox" Classes="add-task-input"
-                 Watermark="Add a task…"
+                 PlaceholderText="Add a task…"
                  Text="{Binding NewTaskTitle, Mode=TwoWay}"
                  VerticalAlignment="Center"
                  Margin="12,0,0,0">