test: подключить vitest и покрыть утилиты юнит-тестами

vitest.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config';
+
+// Kept separate from vite.config.ts so the app build config stays untouched;
+// utility tests run in a plain node environment (no jsdom needed).
+export default defineConfig({
+  test: {
+    include: ['src/**/*.test.{ts,tsx}'],
+    environment: 'node',
+  },
+});