feat: add route selection functionality and improve station handling

- Added `vitest` for testing and created initial tests for route utilities.
- Implemented route selection logic in the App component, allowing users to select start and end stations.
- Updated the NetworkMap component to reflect focused and selected stations, including visual indicators for start and end stations.
- Enhanced the route panel UI to display selected route information and estimated lengths.
- Introduced utility functions for building track adjacency and computing routes based on selected stations.
- Improved styling for route selection and station list items to enhance user experience.

frontend/tsconfig.json

@@ -15,7 +15,7 @@
     "isolatedModules": true,
     "noEmit": true,
     "jsx": "react-jsx",
-    "types": ["vite/client"]
+    "types": ["vite/client", "vitest"]
   },
   "include": ["src"]
 }