feat(03-03): implement StorageCsvExportService

- Replace string.Empty stub with full BuildCsv implementation
- UTF-8 BOM header row: Library,Site,Files,Total Size (MB),Version Size (MB),Last Modified
- RFC 4180 CSV quoting via Csv() helper
- FormatMb() converts bytes to MB with 2 decimal places
- Add explicit System.IO using (required in WPF project)

SharepointToolbox/Services/Export/StorageCsvExportService.cs

@@ -1,14 +1,56 @@
 using SharepointToolbox.Core.Models;
+using System.IO;
+using System.Text;
 
 namespace SharepointToolbox.Services.Export;
 
+/// <summary>
+/// Exports a flat list of StorageNode objects to a UTF-8 BOM CSV.
+/// Compatible with Microsoft Excel (BOM signals UTF-8 encoding).
+/// </summary>
 public class StorageCsvExportService
 {
-    public string BuildCsv(IReadOnlyList<StorageNode> nodes) => string.Empty; // implemented in Plan 03-03
+    public string BuildCsv(IReadOnlyList<StorageNode> nodes)
+    {
+        var sb = new StringBuilder();
+
+        // Header
+        sb.AppendLine("Library,Site,Files,Total Size (MB),Version Size (MB),Last Modified");
+
+        foreach (var node in nodes)
+        {
+            sb.AppendLine(string.Join(",",
+                Csv(node.Name),
+                Csv(node.SiteTitle),
+                node.TotalFileCount.ToString(),
+                FormatMb(node.TotalSizeBytes),
+                FormatMb(node.VersionSizeBytes),
+                node.LastModified.HasValue
+                    ? Csv(node.LastModified.Value.ToString("yyyy-MM-dd"))
+                    : string.Empty));
+        }
+
+        return sb.ToString();
+    }
 
     public async Task WriteAsync(IReadOnlyList<StorageNode> nodes, string filePath, CancellationToken ct)
     {
         var csv = BuildCsv(nodes);
-        await System.IO.File.WriteAllTextAsync(filePath, csv, new System.Text.UTF8Encoding(true), ct);
+        // UTF-8 with BOM for Excel compatibility
+        await File.WriteAllTextAsync(filePath, csv, new UTF8Encoding(encoderShouldEmitUTF8Identifier: true), ct);
+    }
+
+    // ── Helpers ───────────────────────────────────────────────────────────────
+
+    private static string FormatMb(long bytes)
+        => (bytes / (1024.0 * 1024.0)).ToString("F2");
+
+    /// <summary>RFC 4180 CSV field quoting.</summary>
+    private static string Csv(string value)
+    {
+        if (string.IsNullOrEmpty(value)) return string.Empty;
+        if (value.Contains(',') || value.Contains('"') || value.Contains('\n'))
+            return $"\"{value.Replace("\"", "\"\"")}\"";
+        return value;
     }
 }