RepoScanner.Scan() only guarded reparse points on subdirectories found
during recursion, so a configured import root that is itself a junction
walked straight through onto other drives. Apply the same
FileAttributes.ReparsePoint check to the root before scanning.
RepoImportModalViewModel.ScanAndAdd ran the full 5-level recursive scan
synchronously on the UI thread; pointing the folder picker at a broad
directory froze the app. Offload RepoScanner.Scan to Task.Run per folder
and apply results back on the UI thread.
Add a RepoScannerTests case that creates a real junction (mklink /J) at
the scan root and asserts Scan returns empty.