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/vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    include: ['src/**/*.test.ts'],
+    environment: 'node',
+  },
+});