feat(04-06): implement TemplateService and FolderStructureService

- FolderStructureService.CreateFoldersAsync creates folder hierarchy from CSV rows using BulkOperationRunner
- FolderStructureService.BuildUniquePaths deduplicates and sorts paths parent-first by slash depth
- TemplateService already committed; verified compilation and interface compliance
- FolderStructureServiceTests: 4 unit tests pass (BuildUniquePaths edge cases, deduplication, empty levels, BuildPath) + 1 skip
- TemplateServiceTests: 3 unit tests pass (interface impl, SiteTemplate defaults, SiteTemplateOptions defaults) + 2 skip

SharepointToolbox/Services/FolderStructureService.cs

@@ -0,0 +1,69 @@
+using Microsoft.SharePoint.Client;
+using Serilog;
+using SharepointToolbox.Core.Helpers;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Services;
+
+public class FolderStructureService : IFolderStructureService
+{
+    public async Task<BulkOperationSummary<string>> CreateFoldersAsync(
+        ClientContext ctx,
+        string libraryTitle,
+        IReadOnlyList<FolderStructureRow> rows,
+        IProgress<OperationProgress> progress,
+        CancellationToken ct)
+    {
+        // Get library root folder URL
+        var list = ctx.Web.Lists.GetByTitle(libraryTitle);
+        ctx.Load(list, l => l.RootFolder.ServerRelativeUrl);
+        await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, ct);
+
+        var baseUrl = list.RootFolder.ServerRelativeUrl.TrimEnd('/');
+
+        // Build unique folder paths from CSV rows, sorted parent-first
+        var folderPaths = BuildUniquePaths(rows);
+
+        return await BulkOperationRunner.RunAsync(
+            folderPaths,
+            async (path, idx, token) =>
+            {
+                var fullPath = $"{baseUrl}/{path}";
+                ctx.Web.Folders.Add(fullPath);
+                await ExecuteQueryRetryHelper.ExecuteQueryRetryAsync(ctx, progress, token);
+                Log.Information("Created folder: {Path}", fullPath);
+            },
+            progress,
+            ct);
+    }
+
+    /// <summary>
+    /// Builds unique folder paths from CSV rows, sorted parent-first to ensure
+    /// parent folders are created before children.
+    /// </summary>
+    internal static IReadOnlyList<string> BuildUniquePaths(IReadOnlyList<FolderStructureRow> rows)
+    {
+        var paths = new HashSet<string>(StringComparer.OrdinalIgnoreCase);
+
+        foreach (var row in rows)
+        {
+            var parts = new[] { row.Level1, row.Level2, row.Level3, row.Level4 }
+                .Where(s => !string.IsNullOrWhiteSpace(s))
+                .ToArray();
+
+            // Add each level as a path (e.g., "Admin", "Admin/HR", "Admin/HR/Contracts")
+            var current = string.Empty;
+            foreach (var part in parts)
+            {
+                current = string.IsNullOrEmpty(current) ? part.Trim() : $"{current}/{part.Trim()}";
+                paths.Add(current);
+            }
+        }
+
+        // Sort by depth (fewer slashes first) to ensure parent-first ordering
+        return paths
+            .OrderBy(p => p.Count(c => c == '/'))
+            .ThenBy(p => p, StringComparer.OrdinalIgnoreCase)
+            .ToList();
+    }
+}