feat(codechef): implement login and submit

Problem: CodeChef login and submit were stub-only. Language IDs were
also missing for USACO, Kattis, and CodeChef.

Solution: Add StealthySession browser-based login (checks `__NEXT_DATA__`
for current user, fills email/password form) and submit (navigates to
`/{contest_id}/submit/{problem_id}`, selects language via select element
or custom dropdown fallback, sets code via Monaco/CodeMirror/textarea JS,
clicks submit). Cookie cache at `~/.cache/cp-nvim/codechef-cookies.json`.
Add language IDs for all three new platforms to `language_ids.py`.

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",
+    },
 }