A try of overhauling the frontend tests, bailed after 24h+

frontend/vitest.setup.ts

@@ -1 +1,21 @@
-import '@testing-library/jest-dom/vitest';
+import '@testing-library/jest-dom';
+import { vi, beforeAll } from 'vitest';
+
+// Mock localStorage
+const localStorageMock = (() => {
+  let store: Record<string, string> = {};
+  return {
+    getItem: (key: string) => store[key] || null,
+    setItem: (key: string, value: string) => { store[key] = value.toString(); },
+    removeItem: (key: string) => { delete store[key]; },
+    clear: () => { store = {}; },
+  };
+})();
+
+Object.defineProperty(window, 'localStorage', {
+  value: localStorageMock,
+});
+
+beforeAll(() => {
+  vi.clearAllMocks();
+});