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.Windows/WindowsElevatedScanService.cs

@@ -21,6 +21,7 @@ public sealed class WindowsElevatedScanService : IElevatedScanService
         }
     }
 
+    // Queue, index writer, and watchers stay in the user session. Elevation is never requested.
     public bool CanRequestElevation => false;
 
     public string ProtectedContentHint => "Protected content requires administrator privileges.";