fix: mock localStorage in test setup for Zustand persist middleware

frontend/src/stores/__tests__/authStore.test.ts

@@ -3,6 +3,7 @@ import { useAuthStore } from '@/stores/authStore'
 
 describe('authStore', () => {
   beforeEach(() => {
+    localStorage.clear()
     useAuthStore.setState({ token: null, isAuthenticated: false })
   })
 

frontend/src/test/setup.ts

@@ -1 +1,14 @@
 import '@testing-library/jest-dom'
+
+// Zustand persist middleware requires localStorage. jsdom provides it but
+// some test runners strip it — ensure a working implementation is always present.
+const store: Record<string, string> = {}
+Object.defineProperty(globalThis, 'localStorage', {
+  value: {
+    getItem: (k: string) => store[k] ?? null,
+    setItem: (k: string, v: string) => { store[k] = v },
+    removeItem: (k: string) => { delete store[k] },
+    clear: () => { Object.keys(store).forEach((k) => delete store[k]) },
+  },
+  writable: true,
+})