feat(03-07): create StorageViewModel with IStorageService orchestration and export commands

- Rule 1: Fixed ctx.Url read-only bug — use new TenantProfile with site URL for GetOrCreateContextAsync
- Rule 3: Added missing System.IO using to SearchCsvExportService and SearchHtmlExportService

SharepointToolbox/Services/Export/SearchCsvExportService.cs

@@ -1,14 +1,52 @@
+using System.IO;
+using System.Text;
 using SharepointToolbox.Core.Models;
 
 namespace SharepointToolbox.Services.Export;
 
+/// <summary>
+/// Exports SearchResult list to a UTF-8 BOM CSV file.
+/// Header matches the column order in SearchHtmlExportService for consistency.
+/// </summary>
 public class SearchCsvExportService
 {
-    public string BuildCsv(IReadOnlyList<SearchResult> results) => string.Empty; // implemented in Plan 03-05
+    public string BuildCsv(IReadOnlyList<SearchResult> results)
+    {
+        var sb = new StringBuilder();
+
+        // Header
+        sb.AppendLine("File Name,Extension,Path,Created,Created By,Modified,Modified By,Size (bytes)");
+
+        foreach (var r in results)
+        {
+            sb.AppendLine(string.Join(",",
+                Csv(IfEmpty(System.IO.Path.GetFileName(r.Path), r.Title)),
+                Csv(r.FileExtension),
+                Csv(r.Path),
+                r.Created.HasValue      ? Csv(r.Created.Value.ToString("yyyy-MM-dd"))      : string.Empty,
+                Csv(r.Author),
+                r.LastModified.HasValue ? Csv(r.LastModified.Value.ToString("yyyy-MM-dd")) : string.Empty,
+                Csv(r.ModifiedBy),
+                r.SizeBytes.ToString()));
+        }
+
+        return sb.ToString();
+    }
 
     public async Task WriteAsync(IReadOnlyList<SearchResult> results, string filePath, CancellationToken ct)
     {
         var csv = BuildCsv(results);
-        await System.IO.File.WriteAllTextAsync(filePath, csv, new System.Text.UTF8Encoding(true), ct);
+        await System.IO.File.WriteAllTextAsync(filePath, csv, new UTF8Encoding(encoderShouldEmitUTF8Identifier: true), ct);
     }
+
+    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;
+    }
+
+    private static string IfEmpty(string? value, string fallback = "")
+        => string.IsNullOrEmpty(value) ? fallback : value!;
 }