Project: mobx
Task (challenging): Implement user login, logout, and display blog poster's username.

 1) Login API:
POST /api/login
Request: { username: string, password: string }
Response: { success: boolean }
2) On the /login page, add a LoginForm with labels(label[htmlFor="username"], label[htmlFor="password"]), and a .login-submit-btn to submit the form via the Login API.
3) Display the user's username in Header.tsx with the class '.username'.
4) Show the blog author's username with the class '.blog-author'. If blog has no author, display 'Anonymous'.
5) Add a '👋' Logout button (class '.logout-btn') next to the username in Header.tsx. Clicking it logs the user out.
6) Use MobX to manage user states.
Constraint: DO NOT use any third-party packages; ONLY React/MobX APIs are permitted.