Add SharedState singleton observable

StabilityMatrix.Avalonia/DesignData/DesignData.cs

@@ -5,6 +5,7 @@ using System.Collections.ObjectModel;
 using System.Diagnostics.CodeAnalysis;
 using System.Net.Http;
 using Microsoft.Extensions.DependencyInjection;
+using StabilityMatrix.Avalonia.Models;
 using StabilityMatrix.Avalonia.Services;
 using StabilityMatrix.Avalonia.ViewModels;
 using StabilityMatrix.Avalonia.ViewModels.Dialogs;
@@ -62,7 +63,8 @@ public static class DesignData
         services.AddLogging()
             .AddSingleton<IPackageFactory, PackageFactory>()
             .AddSingleton<IUpdateHelper, UpdateHelper>()
-            .AddSingleton<ModelFinder>();
+            .AddSingleton<ModelFinder>()
+            .AddSingleton<SharedState>();
         
         // Mock services
         services

StabilityMatrix.Avalonia/Models/SharedState.cs

@@ -0,0 +1,14 @@
+using CommunityToolkit.Mvvm.ComponentModel;
+
+namespace StabilityMatrix.Avalonia.Models;
+
+/// <summary>
+/// Singleton DI service for observable shared UI state.
+/// </summary>
+public partial class SharedState : ObservableObject
+{
+    /// <summary>
+    /// Whether debug mode enabled from settings page version tap.
+    /// </summary>
+    [ObservableProperty] private bool isDebugMode;
+}