Extract a per-user background host so indexing can later run outside the window.

Keep the GUI as the index writer for now; mutex, named pipe, and opt-in logon autostart prepare Explorer.Host.exe without two SQLite writers.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/Explorer.FileOperations/TransferQueue.cs

@@ -1,3 +1,4 @@
+using Explorer.Contracts;
 using Explorer.Domain;
 using Explorer.Domain.Abstractions;
 using Microsoft.Extensions.Hosting;
@@ -5,7 +6,7 @@ using Microsoft.Extensions.Logging;
 
 namespace Explorer.FileOperations;
 
-public sealed class TransferQueue : BackgroundService
+public sealed class TransferQueue : BackgroundService, ITransferHost
 {
     private readonly IOperationExecutor _executor;
     private readonly IIndexStore _store;