Returns TRUE once an access token is available for the current context:
inside a Shiny app wrapped by server_wrapper(), the per-session token
stored at login; outside Shiny, the CSIAPPS_ACCESS_TOKEN environment
variable.
Details
The check is reactive-friendly: called from a reactive expression or
observer it takes a dependency on the session's token, so a guard like
req(token_ready()) re-fires automatically when login completes, instead of
silently sticking in the cancelled state. Note that make_request() and the
fetch_*() helpers already gate themselves this way, so an explicit guard
is only needed for work that should wait for login without making an API
call (e.g. processing an uploaded file).