543480a4fe
## Problem
Login and submit were stub-only for USACO, Kattis, and CodeChef, leaving
three platforms without a working solve loop.
## Solution
Three commits, one per platform:
**USACO** — httpx-based login via `login-session.php` with cookie cache.
Submit fetches the problem page and parses the form dynamically (action
URL, hidden fields, language select) before POSTing multipart with
`sub_file[]`.
**Kattis** — httpx-based login via `/login/email` (official Kattis CLI
API). Submit is a multipart POST to `/submit`; the `contest` field is
included only when `contest_id != problem_id`. `scrape_contest_list`
URL updated to filter
`kattis_original=on&kattis_recycled=off&user_created=off`.
**CodeChef** — StealthySession browser-based login and submit following
the AtCoder/CF pattern. Login checks `__NEXT_DATA__` for current user
and fills the email/password form. Submit navigates to
`/{contest_id}/submit/{problem_id}`, selects language (standard
`<select>` first, custom dropdown fallback), sets code via
Monaco/CodeMirror/textarea JS, and clicks submit. Retry-on-relogin
pattern matches existing CF behaviour.
Language IDs added to `language_ids.py` for all three platforms.
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|---|---|---|
| .. | ||
| __init__.py | ||
| atcoder.py | ||
| base.py | ||
| codechef.py | ||
| codeforces.py | ||
| cses.py | ||
| kattis.py | ||
| language_ids.py | ||
| models.py | ||
| timeouts.py | ||
| usaco.py | ||