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; }
+}

SharepointToolbox/Core/Models/LogoData.cs

@@ -0,0 +1,7 @@
+namespace SharepointToolbox.Core.Models;
+
+public record LogoData
+{
+    public string Base64 { get; init; } = string.Empty;
+    public string MimeType { get; init; } = string.Empty;
+}

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; }
 }

SharepointToolbox/Infrastructure/Persistence/BrandingRepository.cs

@@ -0,0 +1,74 @@
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Infrastructure.Persistence;
+
+public class BrandingRepository
+{
+    private readonly string _filePath;
+    private readonly SemaphoreSlim _writeLock = new(1, 1);
+
+    public BrandingRepository(string filePath)
+    {
+        _filePath = filePath;
+    }
+
+    public async Task<BrandingSettings> LoadAsync()
+    {
+        if (!File.Exists(_filePath))
+            return new BrandingSettings();
+
+        string json;
+        try
+        {
+            json = await File.ReadAllTextAsync(_filePath, Encoding.UTF8);
+        }
+        catch (IOException ex)
+        {
+            throw new InvalidDataException($"Failed to read branding file: {_filePath}", ex);
+        }
+
+        try
+        {
+            var settings = JsonSerializer.Deserialize<BrandingSettings>(json,
+                new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
+            return settings ?? new BrandingSettings();
+        }
+        catch (JsonException ex)
+        {
+            throw new InvalidDataException($"Branding file contains invalid JSON: {_filePath}", ex);
+        }
+    }
+
+    public async Task SaveAsync(BrandingSettings settings)
+    {
+        await _writeLock.WaitAsync();
+        try
+        {
+            var json = JsonSerializer.Serialize(settings,
+                new JsonSerializerOptions
+                {
+                    WriteIndented = true,
+                    PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+                });
+
+            var tmpPath = _filePath + ".tmp";
+            var dir = Path.GetDirectoryName(_filePath);
+            if (!string.IsNullOrEmpty(dir))
+                Directory.CreateDirectory(dir);
+
+            await File.WriteAllTextAsync(tmpPath, json, Encoding.UTF8);
+
+            // Validate round-trip before replacing
+            JsonDocument.Parse(await File.ReadAllTextAsync(tmpPath, Encoding.UTF8)).Dispose();
+
+            File.Move(tmpPath, _filePath, overwrite: true);
+        }
+        finally
+        {
+            _writeLock.Release();
+        }
+    }
+}