feat(04-03): implement FileTransferService with MoveCopyUtil and conflict policies

- FileTransferService.cs: CSOM copy/move via MoveCopyUtil.CopyFileByPath/MoveFileByPath
- Conflict policies: Skip (catch ServerException), Overwrite (overwrite=true), Rename (KeepBoth=true)
- ResourcePath.FromDecodedUrl for special character support
- Recursive folder enumeration with system folder filtering
- EnsureFolderAsync creates intermediate destination folders
- Best-effort metadata preservation (ResetAuthorAndCreatedOnCopy=false)
- FileTransferServiceTests.cs: 4 passing tests, 3 skipped (integration)

SharepointToolbox.Tests/Services/FileTransferServiceTests.cs

@@ -0,0 +1,57 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Services;
+
+namespace SharepointToolbox.Tests.Services;
+
+public class FileTransferServiceTests
+{
+    [Fact]
+    public void FileTransferService_Implements_IFileTransferService()
+    {
+        var service = new FileTransferService();
+        Assert.IsAssignableFrom<IFileTransferService>(service);
+    }
+
+    [Fact]
+    public void TransferJob_DefaultValues_AreCorrect()
+    {
+        var job = new TransferJob();
+        Assert.Equal(TransferMode.Copy, job.Mode);
+        Assert.Equal(ConflictPolicy.Skip, job.ConflictPolicy);
+    }
+
+    [Fact]
+    public void ConflictPolicy_HasAllValues()
+    {
+        Assert.Equal(3, Enum.GetValues<ConflictPolicy>().Length);
+        Assert.Contains(ConflictPolicy.Skip, Enum.GetValues<ConflictPolicy>());
+        Assert.Contains(ConflictPolicy.Overwrite, Enum.GetValues<ConflictPolicy>());
+        Assert.Contains(ConflictPolicy.Rename, Enum.GetValues<ConflictPolicy>());
+    }
+
+    [Fact]
+    public void TransferMode_HasAllValues()
+    {
+        Assert.Equal(2, Enum.GetValues<TransferMode>().Length);
+        Assert.Contains(TransferMode.Copy, Enum.GetValues<TransferMode>());
+        Assert.Contains(TransferMode.Move, Enum.GetValues<TransferMode>());
+    }
+
+    [Fact(Skip = "Requires live SharePoint tenant")]
+    public async Task TransferAsync_CopyMode_CopiesFiles()
+    {
+        // Integration test — needs real ClientContext
+    }
+
+    [Fact(Skip = "Requires live SharePoint tenant")]
+    public async Task TransferAsync_MoveMode_DeletesSourceAfterCopy()
+    {
+        // Integration test — needs real ClientContext
+    }
+
+    [Fact(Skip = "Requires live SharePoint tenant")]
+    public async Task TransferAsync_SkipConflict_DoesNotOverwrite()
+    {
+        // Integration test — needs real ClientContext
+    }
+}