feat(10-01): create BrandingService with magic byte validation and auto-compression

- Add IBrandingService interface with ImportLogoAsync, Save/Clear/GetMspLogoAsync
- Add BrandingService: PNG/JPEG magic byte detection, rejects unsupported formats with
  descriptive error, auto-compresses files over 512 KB using WPF PresentationCore imaging
- Add BrandingServiceTests: 9 tests covering validation, rejection, compression, CRUD
- Deviation: used WPF BitmapEncoder/TransformedBitmap instead of System.Drawing.Bitmap
  (System.Drawing.Common not available without new NuGet package; WPF PresentationCore
  is in the existing stack per architectural decisions)

SharepointToolbox/Services/IBrandingService.cs

@@ -0,0 +1,11 @@
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Services;
+
+public interface IBrandingService
+{
+    Task<LogoData> ImportLogoAsync(string filePath);
+    Task SaveMspLogoAsync(LogoData logo);
+    Task ClearMspLogoAsync();
+    Task<LogoData?> GetMspLogoAsync();
+}