feat(tests): add frontend automated testing setup with Vitest and jsdom

tests/frontend/setup.js

@@ -0,0 +1,16 @@
+import { afterEach, beforeEach } from 'vitest';
+
+beforeEach(() => {
+  // Ensure storage is clean before each test to avoid cross-test pollution
+  localStorage.clear();
+  sessionStorage.clear();
+
+  // Reset DOM state for modules that rely on body attributes
+  document.body.innerHTML = '';
+  document.body.dataset.page = '';
+});
+
+afterEach(() => {
+  // Clean any dynamically attached globals by tests
+  delete document.body.dataset.page;
+});