added testing

frontend/vitest.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config';
+
+// Pure-logic unit tests only (no Astro/DOM). Component/integration tests, if
+// added later, should switch to `getViteConfig` from 'astro/config'.
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+  },
+});