feat: implement login and submit for USACO, Kattis, and CodeChef (#325)

## 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.

scrapers/language_ids.py

@@ -11,6 +11,18 @@ LANGUAGE_IDS = {
         "cpp": "C++17",
         "python": "Python3",
     },
+    "usaco": {
+        "cpp": "cpp",
+        "python": "python",
+    },
+    "kattis": {
+        "cpp": "C++17",
+        "python": "Python 3",
+    },
+    "codechef": {
+        "cpp": "C++ 17",
+        "python": "Python 3",
+    },
 }