feat(10-01): create logo models, BrandingRepository, and repository tests

- Add LogoData record with Base64 and MimeType init properties
- Add BrandingSettings class with nullable MspLogo property
- Extend TenantProfile with nullable ClientLogo property (additive)
- Add BrandingRepository mirroring SettingsRepository pattern (write-then-replace)
- Add BrandingRepositoryTests: 5 tests covering load defaults, round-trip, dir creation, and TenantProfile serialization

SharepointToolbox/Core/Models/BrandingSettings.cs

@@ -0,0 +1,6 @@
+namespace SharepointToolbox.Core.Models;
+
+public class BrandingSettings
+{
+    public LogoData? MspLogo { get; set; }
+}