Problem: CF and AtCoder always did a full browser login on every
`login` invocation, even with valid cookies. AtCoder submit never
persisted cookies, re-logging in on every submit. CF's cookie guard
used `X-User-Handle` (no longer set by CF — now `X-User-Sha1`),
so cookies were never saved. CF `login_action` was missing
`wait_for_selector` for the form that appears after the Cloudflare
gate reloads. AtCoder submit injected source via CodeMirror which
doesn't exist on AtCoder (it uses ACE editor).
Solution: Added cookie fast paths to CF and AtCoder login — emit
`checking_login` and return early if the existing session is valid.
`checking_login` is only emitted when cookies actually exist; fresh
starts go straight to `logging_in`. Fixed CF cookie guard to
`X-User-Sha1` and added `wait_for_selector` for the login form.
Rewrote AtCoder submit to use `set_input_files` on the real source
file path, with `wait_for_function` on `#plain-textarea` to confirm
the ACE editor populated before clicking submit.
