Add frontend testing infrastructure with Vitest (#14)

frontend/vite.config.ts

@@ -1,3 +1,4 @@
+/// <reference types="vitest" />
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 
@@ -9,5 +10,11 @@ export default defineConfig({
       '/health': 'http://localhost:8080',
       '/project': 'http://localhost:8080',
     }
+  },
+  test: {
+    globals: true,
+    environment: 'jsdom',
+    setupFiles: './src/test/setup.ts',
+    css: true,
   }
 })