🧪 [testing improvement description] Implement unit tests for useAuthStore and fix linting.

🎯 **What:** This PR adds comprehensive unit tests for the `useAuthStore` Zustand store and resolves a CI linting failure by downgrading ESLint to version 9.
📊 **Coverage:** The new tests cover:
- Initial state verification.
- User state updates via `setUser`.
- Token management via `setToken`.
- Loading state toggles with `setLoading`.
- Complete logout flow, including `localStorage` cleanup.
✨ **Result:** Increased reliability of the authentication logic and restored CI health. The store implementation was also refined to properly include and handle the `token` property.

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

frontend/package.json

@@ -22,7 +22,7 @@
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",
-    "eslint": "^10",
+    "eslint": "^9",
     "eslint-config-next": "16.1.7",
     "tailwindcss": "^4",
     "typescript": "^5"