Clean up frontend template macros.
Frontend add login & register forms.
Backend: handle registration & login via the API.
API: load user from 'Authorization' header before each request. Add custom 'login_required' decorator.
Connect frontend 'login' & 'register' views to API and store user's key in session.
Admin: handle authentication from frontend session cookie.
Add user dropdown to navbar. Handle logout.
Handle errors gracefully in frontend.
Only put Procurement detail page behind login.