Add basic JS tests

web/vitest.config.js

@@ -0,0 +1,14 @@
+/* eslint-disable import/no-extraneous-dependencies */
+import { defineConfig } from "vitest/config";
+
+// Standalone config (separate from vite.config.js) so the PWA plugin isn't loaded during tests.
+// Tests run in the default "node" environment -- see src/test/setup.js for the minimal window stub
+// that lets the pure-logic modules import without jsdom.
+export default defineConfig({
+  test: {
+    environment: "node",
+    setupFiles: ["./src/test/setup.js"],
+    include: ["src/**/*.test.{js,jsx}"],
+    globals: false, // explicit `import { describe, it, expect } from "vitest"` -> no eslint env change
+  },
+});