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/Controls/ThemedDatePicker.axaml

@@ -154,7 +154,7 @@
                        Foreground="{DynamicResource TextDimBrush}"
                        Margin="0,0,8,0"/>
             <TextBox Grid.Column="1" x:Name="TimeInput"
-                     Watermark="HH:mm" MaxLength="5"
+                     PlaceholderText="HH:mm" MaxLength="5"
                      Text="{Binding #Root.TimeText, Mode=TwoWay}"/>
             <Button Grid.Column="2" Content="Done"
                     Click="OnDoneClick"