Fix ANR: only set SelectedNode for leaf nodes, not directories

Directory/agent taps now only toggle expand/collapse without triggering
heavy MCP data loading via GenerateAndNavigate.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sharpninja

src/RequestTracker.Core/ViewModels/MainWindowViewModel.cs

@@ -240,6 +240,7 @@ private void TreeItemTapped(FileNode? node)
                     }
                 }
             }
+            return;
         }
         SelectedNode = node;
     }