[auth]: update login page, add login button, add token refresh

[amahuli03]

Description

• Update the login page, header, and session management for non-admin users now that they can register and activate accounts.
• The login page currently has a warning that says "This login is for Code for Philly administrators" and has no links to register or reset password. With regular users now able to create accounts, the page needs to reflect that.
• Sessions currently expire silently after 60 minutes (the access token lifetime) with no refresh mechanism. The backend already supports token refresh via /api/auth/jwt/refresh/ — the frontend needs an Axios interceptor that catches 401 responses and attempts a silent refresh before logging the user out.
• The header currently only shows a sign-out link for logged-in users. Unauthenticated users have no way to get to the login page from the navigation (they'd have to know the URL). We can fix this by adding a login/signup button on the main menu.

Blocked by #485