feat(04-08): create TransferViewModel and TransferView
- TransferViewModel: source/dest site selection, transfer mode, conflict policy, confirmation dialog, per-item results, failed-items CSV export
- TransferView.xaml: DockPanel layout with GroupBoxes for source/dest, mode radio buttons, conflict policy ComboBox, progress bar, cancel button, export failed items button
- TransferView.xaml.cs: code-behind wires SitePickerDialog + FolderBrowserDialog for source and dest browsing
- Added EnumBoolConverter and StringToVisibilityConverter to IndentConverter.cs
- Registered converters in App.xaml; registered TransferViewModel, TransferView, IFileTransferService, BulkResultCsvExportService in App.xaml.cs
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