Start the work of clearly distingushing client side types, server side
types and types shared over the API by renaming "AccountSession" and
"Session" names used on the server to "ServerSession".
Provide a basic account system with login and server side session store
identified by a cookie. Upon successful login a signed session cookie
is set by the server with the session stored on the server identifying
which account it is logged in as. The client uses a shared useFetch on
the session endpoint to identify if it's logged in and which account it
is logged in as, and refreshes this when loggin in or out.
