Login and registration screens

[Filipejqueiroga]

Authentication forms that connect to the POST /auth/login and POST /auth/register endpoints. The JWT token received is saved to localStorage for use in subsequent requests.

Acceptance criteria

• Login screen with email and password fields
• Registration screen with name, email, password, and role selection (Buyer / Seller)
• JWT token saved to localStorage after a successful login
• Redirect to index.html after login
• Error message displayed for invalid credentials
• Logout button clears the token and redirects to the login screen
• Navbar displays the user's name when logged in