fix(ui): reparse-check repo-scan root folder and move scan off UI thread

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.

src/ClaudeDo.Ui/Services/RepoScanner.cs

@@ -16,6 +16,15 @@ public static class RepoScanner
         if (string.IsNullOrWhiteSpace(parentFolder) || !Directory.Exists(parentFolder))
             return Array.Empty<RepoCandidate>();
 
+        try
+        {
+            var info = new DirectoryInfo(parentFolder);
+            if (info.Attributes.HasFlag(FileAttributes.ReparsePoint))
+                return Array.Empty<RepoCandidate>();
+        }
+        catch (Exception e) when (e is IOException or UnauthorizedAccessException)
+        { return Array.Empty<RepoCandidate>(); }
+
         var result = new List<RepoCandidate>();
         ScanDirectory(parentFolder, depth: 0, result);
         return result;