Add MockDiscordRichPresenceService for design time

StabilityMatrix.Avalonia/DesignData/DesignData.cs

@@ -81,7 +81,8 @@ public static class DesignData
             .AddSingleton<INotificationService, MockNotificationService>()
             .AddSingleton<ISharedFolders, MockSharedFolders>()
             .AddSingleton<IDownloadService, MockDownloadService>()
-            .AddSingleton<IHttpClientFactory, MockHttpClientFactory>();
+            .AddSingleton<IHttpClientFactory, MockHttpClientFactory>()
+            .AddSingleton<IDiscordRichPresenceService, MockDiscordRichPresenceService>();
         
         // Placeholder services that nobody should need during design time
         services

StabilityMatrix.Avalonia/DesignData/MockDiscordRichPresenceService.cs

@@ -0,0 +1,18 @@
+using System;
+using StabilityMatrix.Avalonia.Services;
+
+namespace StabilityMatrix.Avalonia.DesignData;
+
+public class MockDiscordRichPresenceService : IDiscordRichPresenceService
+{
+    /// <inheritdoc />
+    public void Dispose()
+    {
+        GC.SuppressFinalize(this);
+    }
+
+    /// <inheritdoc />
+    public void UpdateState()
+    {
+    }
+}