feat(taskbar): Remove WPF TaskbarItemInfo dead code [P2-015] (#81)

* feat(taskbar): Remove WPF TaskbarItemInfo dead code [P2-015]

- Delete StatusToTaskbarStateConverter.cs: converter was already migrated
  to Avalonia IValueConverter returning null (no-op stub), and was not
  referenced by any XAML or C# consumer
- Remove empty InitTaskbarInfo() stub and its [SupportedOSPlatform("windows")]
  guard from MainWindow.xaml.cs: no taskbar progress implementation exists;
  accepting silent omission per the issue acceptance criteria
- Remove now-unused 'using System.Runtime.Versioning' from MainWindow.xaml.cs

No System.Windows.Shell references remain anywhere in the codebase.
App does not crash on Linux. Taskbar progress is silently omitted.

Closes #27

* Update outdated comment

FModel/MainWindow.xaml

@@ -16,7 +16,8 @@
     <!--
         Title is set in code-behind (OnLoaded / property change) to replicate the
         DataTrigger-based title logic that was in the WPF AdonisWindow style.
-        TaskbarItemInfo (Windows-only) is handled in code-behind with an OS guard.
+        TaskbarItemInfo (Windows-only) has no Avalonia equivalent; taskbar progress
+        is silently omitted.
     -->
     <Grid>
         <Grid.RowDefinitions>

FModel/MainWindow.xaml.cs

@@ -1,7 +1,6 @@
 using System;
 using System.ComponentModel;
 using System.Linq;
-using System.Runtime.Versioning;
 using System.Threading.Tasks;
 using Avalonia;
 using Avalonia.Controls;
@@ -47,16 +46,6 @@ public partial class MainWindow : Window
         _applicationView.PropertyChanged += OnApplicationViewPropertyChanged;
         UpdateWindowTitle();
 
-        // Windows-only: set up TaskbarItemInfo progress
-        if (OperatingSystem.IsWindows())
-            InitTaskbarInfo();
-    }
-
-    [SupportedOSPlatform("windows")]
-    private void InitTaskbarInfo()
-    {
-        // TODO(P2-015): set up Windows taskbar progress indicator once
-        // StatusToTaskbarStateConverter is migrated.
     }
 
     private void UpdateWindowTitle()

FModel/Views/Resources/Converters/StatusToTaskbarStateConverter.cs

@@ -1,24 +0,0 @@
-using System;
-using System.Globalization;
-using Avalonia.Data.Converters;
-
-namespace FModel.Views.Resources.Converters;
-
-/// <summary>
-/// Stub: Avalonia has no cross-platform TaskbarItemProgressState equivalent.
-/// Returns null; actual taskbar integration is tracked by TODO(P2-015).
-/// Previously mapped EStatusKind to System.Windows.Shell.TaskbarItemProgressState.
-/// </summary>
-public class StatusToTaskbarStateConverter : IValueConverter
-{
-    public static readonly StatusToTaskbarStateConverter Instance = new();
-
-    public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
-    {
-        // TODO(P2-015): Implement via Avalonia taskbar API when available.
-        return null;
-    }
-
-    public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
-        => throw new NotImplementedException();
-}