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>

tests/Explorer.Application.Tests/WorkbenchHostTests.cs

@@ -0,0 +1,60 @@
+using Explorer.Application;
+using Explorer.Contracts;
+using Explorer.Domain;
+
+namespace Explorer.Application.Tests;
+
+public class WorkbenchHostTests
+{
+    [Fact]
+    public void Forwards_indexing_and_transfer_sessions()
+    {
+        var indexing = new FakeIndexing();
+        var transfers = new FakeTransfers();
+        IWorkbenchHost host = new WorkbenchHost(indexing, transfers);
+        Assert.Same(indexing, host.Indexing);
+        Assert.Same(transfers, host.Transfers);
+
+        host.Indexing.EnqueueFullScan(7);
+        host.Transfers.PauseAll();
+        Assert.Equal(7, indexing.FullScanId);
+        Assert.True(transfers.PausedAll);
+    }
+
+    private sealed class FakeIndexing : IIndexingHost
+    {
+        public long FullScanId { get; private set; }
+        public event EventHandler<ScanProgress>? ProgressChanged;
+
+        public void EnqueueFullScan(long sourceId) => FullScanId = sourceId;
+        public void EnqueueFolderScan(long sourceId, string pathRel) { }
+        public void EnqueueReconcile(long sourceId, string pathRel) { }
+        public void Cancel(long sourceId) { }
+
+        public void Raise() => ProgressChanged?.Invoke(this, new ScanProgress
+        {
+            SourceId = 1,
+            CurrentPath = "",
+            Status = ScanJobStatus.Running
+        });
+    }
+
+    private sealed class FakeTransfers : ITransferHost
+    {
+        public bool PausedAll { get; private set; }
+        public event EventHandler? Changed = delegate { };
+        public event EventHandler<TransferJob>? JobFinished = delegate { };
+        public bool IsPaused => PausedAll;
+        public IReadOnlyList<TransferJob> Snapshot() => [];
+        public void PauseAll() => PausedAll = true;
+        public void ResumeAll() => PausedAll = false;
+        public void Pause(long jobId) { }
+        public void Resume(long jobId) { }
+        public void Retry(long jobId) { }
+        public void Cancel(long jobId) { }
+        public void Dismiss(long jobId) { }
+        public void ClearFinished() { }
+        public bool MoveUp(long jobId) => false;
+        public bool MoveDown(long jobId) => false;
+    }
+}