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/DiffModalView.axaml

@@ -5,7 +5,7 @@
         x:DataType="vm:DiffModalViewModel"
         Title="Diff"
         Width="1200" Height="800"
-        SystemDecorations="None"
+        WindowDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         WindowStartupLocation="CenterOwner"
         Background="{StaticResource SurfaceBrush}">

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>

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

@@ -6,7 +6,7 @@
         Title="Merge worktree"
         Width="560" Height="460"
         CanResize="False"
-        SystemDecorations="None"
+        WindowDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">

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">

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

@@ -6,7 +6,7 @@
         Title="Unfinished planning session"
         Width="440" Height="200"
         CanResize="False"
-        SystemDecorations="None"
+        WindowDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}">

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

@@ -8,7 +8,7 @@
         Width="1100" Height="720"
         MinWidth="640" MinHeight="400"
         WindowStartupLocation="CenterOwner"
-        SystemDecorations="BorderOnly"
+        WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1"
         Background="Transparent"

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

@@ -9,7 +9,7 @@
         CanResize="True"
         WindowStartupLocation="CenterOwner"
         Background="{DynamicResource SurfaceBrush}"
-        SystemDecorations="BorderOnly"
+        WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1">