feat(11-01): add ReportBranding model and BrandingHtmlHelper with tests

- Add ReportBranding positional record bundling MspLogo and ClientLogo
- Add BrandingHtmlHelper static class generating flex branding header HTML
- Add BrandingHtmlHelperTests covering all 4 logo states (null, both null, single, both)
- Add InternalsVisibleTo for SharepointToolbox.Tests in project file

SharepointToolbox/Core/Models/ReportBranding.cs

@@ -0,0 +1,8 @@
+namespace SharepointToolbox.Core.Models;
+
+/// <summary>
+/// Bundles MSP and client logos for passing to export services.
+/// Export services receive this as a simple DTO — they don't know
+/// about IBrandingService or ProfileService.
+/// </summary>
+public record ReportBranding(LogoData? MspLogo, LogoData? ClientLogo);

SharepointToolbox/Services/Export/BrandingHtmlHelper.cs

@@ -0,0 +1,37 @@
+using System.Text;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Services.Export;
+
+/// <summary>
+/// Generates the branding header HTML fragment for HTML reports.
+/// Called by each HTML export service between &lt;body&gt; and &lt;h1&gt;.
+/// Returns empty string when no logos are configured (no broken images).
+/// </summary>
+internal static class BrandingHtmlHelper
+{
+    public static string BuildBrandingHeader(ReportBranding? branding)
+    {
+        if (branding is null) return string.Empty;
+
+        var msp = branding.MspLogo;
+        var client = branding.ClientLogo;
+
+        if (msp is null && client is null) return string.Empty;
+
+        var sb = new StringBuilder();
+        sb.AppendLine("<div style=\"display:flex;gap:16px;align-items:center;padding:12px 24px 0;\">");
+
+        if (msp is not null)
+            sb.AppendLine($"  <img src=\"data:{msp.MimeType};base64,{msp.Base64}\" alt=\"\" style=\"max-height:60px;max-width:200px;object-fit:contain;\">");
+
+        if (msp is not null && client is not null)
+            sb.AppendLine("  <div style=\"flex:1\"></div>");
+
+        if (client is not null)
+            sb.AppendLine($"  <img src=\"data:{client.MimeType};base64,{client.Base64}\" alt=\"\" style=\"max-height:60px;max-width:200px;object-fit:contain;\">");
+
+        sb.AppendLine("</div>");
+        return sb.ToString();
+    }
+}

SharepointToolbox/SharepointToolbox.csproj

@@ -18,6 +18,12 @@
     <IncludeNativeLibrariesForSelfExtract>true</IncludeNativeLibrariesForSelfExtract>
   </PropertyGroup>
 
+  <ItemGroup>
+    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleToAttribute">
+      <_Parameter1>SharepointToolbox.Tests</_Parameter1>
+    </AssemblyAttribute>
+  </ItemGroup>
+
   <ItemGroup>
     <ApplicationDefinition Remove="App.xaml" />
     <Page Include="App.xaml" />