feat(01-01): create WPF solution with Generic Host entry point and NuGet packages

- SharepointToolbox.slnx solution with WPF project
- net10.0-windows target, PublishTrimmed=false, StartupObject set
- App.xaml StartupUri removed, App demoted from ApplicationDefinition to Page
- App.xaml.cs: [STAThread] Main with Host.CreateDefaultBuilder + Serilog rolling file
- All NuGet packages: CommunityToolkit.Mvvm 8.4.2, MSAL 4.83.3, PnP.Framework 1.18.0, Serilog 4.3.1
- Build: 0 errors, 0 warnings

SharepointToolbox/App.xaml.cs

@@ -0,0 +1,40 @@
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Serilog;
+using System.Windows;
+
+namespace SharepointToolbox;
+
+/// <summary>
+/// Interaction logic for App.xaml
+/// </summary>
+public partial class App : Application
+{
+    [STAThread]
+    public static void Main(string[] args)
+    {
+        using IHost host = Host.CreateDefaultBuilder(args)
+            .UseSerilog((ctx, cfg) => cfg
+                .WriteTo.File(
+                    System.IO.Path.Combine(
+                        Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+                        "SharepointToolbox", "logs", "app-.log"),
+                    rollingInterval: RollingInterval.Day,
+                    retainedFileCountLimit: 30))
+            .ConfigureServices(RegisterServices)
+            .Build();
+
+        host.Start();
+        App app = new();
+        app.InitializeComponent();
+        app.MainWindow = host.Services.GetRequiredService<MainWindow>();
+        app.MainWindow.Visibility = Visibility.Visible;
+        app.Run();
+    }
+
+    private static void RegisterServices(HostBuilderContext ctx, IServiceCollection services)
+    {
+        // Placeholder — services registered in subsequent plans
+        services.AddSingleton<MainWindow>();
+    }
+}