Added a new `EnvelopeSender.razor` component with the route `/sender`.
Updated the route for `LoginSender.razor` from `/login` to `/sender/login`.
Modified navigation logic in `LoginSender.razor` to redirect to
`/sender` after a successful login instead of the root route.
Implemented a new `LoginSender.razor` component to provide a login page for "Sender" users. The page includes:
- A responsive card layout with a header, input fields for "Username" and "Password," and a submit button with a loading spinner.
- Error handling for invalid credentials and server errors, with appropriate alerts.
- A password visibility toggle for better user experience.
- Integration with `AuthService` for authentication and `NavigationManager` for redirection.
Added Blazor code-behind logic to manage state, handle login submission, and trigger login on "Enter" key press. The page is styled with a custom theme and includes a footer for support information.
