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/Services/InstallerLocator.cs

@@ -7,7 +7,8 @@ public sealed class InstallerLocator
     private const string UninstallerSubdir = "uninstaller";
 
     public string? Find()
-        => FindByWalkingUp(AppContext.BaseDirectory) ?? FindByRegistry();
+        => FindByWalkingUp(AppContext.BaseDirectory)
+           ?? (OperatingSystem.IsWindows() ? FindByRegistry() : null);
 
     public string? FindByWalkingUp(string startDir)
     {

src/ClaudeDo.Ui/ViewModels/IslandsShellViewModel.cs

@@ -29,8 +29,8 @@ public sealed partial class IslandsShellViewModel : ViewModelBase
 
     public bool IsOffline => Worker?.IsConnected != true && Worker?.IsReconnecting != true;
 
-    private readonly UpdateCheckService _updateCheck;
+    private readonly UpdateCheckService _updateCheck = null!;
-    private readonly InstallerLocator _installerLocator;
+    private readonly InstallerLocator _installerLocator = null!;
     private readonly IDbContextFactory<ClaudeDoDbContext>? _dbFactory;
     private readonly Func<WorktreesOverviewModalViewModel> _worktreesOverviewVmFactory = () => null!;
     private readonly Func<MergeModalViewModel> _mergeVmFactory = () => null!;
@@ -297,17 +297,7 @@ public sealed partial class IslandsShellViewModel : ViewModelBase
         RestartWorkerStatus = "Restarting worker…";
         try
         {
-            await Task.Run(() =>
+            await Task.Run(RestartWorkerService);
-            {
-                using var sc = new System.ServiceProcess.ServiceController("ClaudeDoWorker");
-                if (sc.Status != System.ServiceProcess.ServiceControllerStatus.Stopped)
-                {
-                    sc.Stop();
-                    sc.WaitForStatus(System.ServiceProcess.ServiceControllerStatus.Stopped, TimeSpan.FromSeconds(20));
-                }
-                sc.Start();
-                sc.WaitForStatus(System.ServiceProcess.ServiceControllerStatus.Running, TimeSpan.FromSeconds(20));
-            });
             await FlashRestartStatusAsync("Worker restarted.");
         }
         catch (InvalidOperationException)
@@ -321,6 +311,19 @@ public sealed partial class IslandsShellViewModel : ViewModelBase
         }
     }
 
+    [System.Runtime.Versioning.SupportedOSPlatform("windows")]
+    private static void RestartWorkerService()
+    {
+        using var sc = new System.ServiceProcess.ServiceController("ClaudeDoWorker");
+        if (sc.Status != System.ServiceProcess.ServiceControllerStatus.Stopped)
+        {
+            sc.Stop();
+            sc.WaitForStatus(System.ServiceProcess.ServiceControllerStatus.Stopped, TimeSpan.FromSeconds(20));
+        }
+        sc.Start();
+        sc.WaitForStatus(System.ServiceProcess.ServiceControllerStatus.Running, TimeSpan.FromSeconds(20));
+    }
+
     private async Task FlashRestartStatusAsync(string text)
     {
         RestartWorkerStatus = text;

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"

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">

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -10,7 +10,7 @@
         Background="{DynamicResource VoidBrush}"
         Icon="avares://ClaudeDo.Ui/Assets/ClaudeTask.ico"
         CanResize="True"
-        SystemDecorations="BorderOnly"
+        WindowDecorations="BorderOnly"
         ExtendClientAreaToDecorationsHint="True"
         ExtendClientAreaTitleBarHeightHint="-1">
   <Window.Resources>

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">
 

src/ClaudeDo.Ui/Views/Planning/ConflictResolutionView.axaml

@@ -5,7 +5,7 @@
         x:Class="ClaudeDo.Ui.Views.Planning.ConflictResolutionView"
         Title="Merge conflict"
         Width="560" SizeToContent="Height"
-        SystemDecorations="None"
+        WindowDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         WindowStartupLocation="CenterOwner"
         Background="{StaticResource SurfaceBrush}">

src/ClaudeDo.Ui/Views/Planning/PlanningDiffView.axaml

@@ -5,7 +5,7 @@
         x:DataType="vm:PlanningDiffViewModel"
         Title="Planning — Combined diff"
         Width="1100" Height="700"
-        SystemDecorations="None"
+        WindowDecorations="None"
         ExtendClientAreaToDecorationsHint="True"
         WindowStartupLocation="CenterOwner"
         Background="{StaticResource SurfaceBrush}">