fix(02-01): add export service stubs and fix PermissionsService compile errors

[Rule 3 - Blocking] CsvExportService/HtmlExportService stubs added so export test
files compile. [Rule 1 - Bug] PermissionsService: removed Principal.Email (not on
Principal, only on User) and changed folder param from Folder to ListItem (SecurableObject).

SharepointToolbox/Services/Export/HtmlExportService.cs

@@ -0,0 +1,18 @@
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Services.Export;
+
+/// <summary>
+/// Exports permission entries to HTML format.
+/// Full implementation will be added in Plan 03.
+/// </summary>
+public class HtmlExportService
+{
+    /// <summary>Builds an HTML string from the supplied permission entries.</summary>
+    public string BuildHtml(IReadOnlyList<PermissionEntry> entries) =>
+        throw new NotImplementedException("HtmlExportService.BuildHtml — implemented in Plan 03");
+
+    /// <summary>Writes the HTML output to a file.</summary>
+    public Task WriteAsync(IReadOnlyList<PermissionEntry> entries, string filePath, CancellationToken ct) =>
+        throw new NotImplementedException("HtmlExportService.WriteAsync — implemented in Plan 03");
+}