Project: mobx
Task (moderate): 1) Add stores/route.ts to control the Route Logic
The definition of state is:
```
class RouteStore {
  currentRoute: string;
}
```
The initial state of route will be the pathname of the current page.
2) In stores/route.ts, listens to browser history change, and updates the currentRoute in the store.
3) When path is '/', shows Main.tsx, when path is '/login', shows pages/Login.tsx. Header is visible for every page.
4) Render <h1>User Login</h1> in components/Login.tsx. Add Button with text '🔑' in the right of Header.tsx, when clicked, go to '/login'.
Constraint: DO NOT USE any third-party packages, ONLY React/MobX APIs can be used to optimize performance.