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/TenantProfile.cs

@@ -5,4 +5,5 @@ public class TenantProfile
     public string Name { get; set; } = string.Empty;
     public string TenantUrl { get; set; } = string.Empty;
     public string ClientId { get; set; } = string.Empty;
+    public LogoData? ClientLogo { get; set; }
 }