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/SettingsModalView.axaml

@@ -8,7 +8,7 @@
         x:DataType="vm:SettingsModalViewModel"
         Title="Settings"
         Width="580" Height="760"
-        SystemDecorations="None"
+        WindowDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">
@@ -94,7 +94,7 @@
                 <StackPanel Spacing="4">
                   <TextBlock Classes="field-label" Text="Default instructions"/>
                   <TextBox AcceptsReturn="True" TextWrapping="Wrap" Height="110"
-                           Watermark="Baseline instructions applied to every task"
+                           PlaceholderText="Baseline instructions applied to every task"
                            Text="{Binding General.DefaultClaudeInstructions, Mode=TwoWay}"/>
                 </StackPanel>
                 <Grid ColumnDefinitions="*,12,*,12,*">
@@ -133,7 +133,7 @@
                   <StackPanel Grid.Column="2" Spacing="4">
                     <TextBlock Classes="field-label" Text="Central worktree root"/>
                     <TextBox Text="{Binding Worktrees.CentralWorktreeRoot, Mode=TwoWay}"
-                             Watermark="e.g. C:\worktrees"/>
+                             PlaceholderText="e.g. C:\worktrees"/>
                   </StackPanel>
                 </Grid>
                 <StackPanel Orientation="Horizontal" Spacing="8">