fix(kattis): fix login, language ID, and submit path

Problem: Kattis login used `/login/email` without a CSRF token (always
failed), the homepage login check used GET requests that Kattis blocks
from httpx, the C++ language ID was `"C++17"` (rejected by the API),
and `submit.lua` passed a relative source file path to Python whose cwd
is the plugin directory.

Solution: Switch login to `POST /login` with `script=true` (200 =
success, 403 = bad credentials), remove the broken `_check_kattis_login`
entirely, add a submit retry on `"Request validation failed"`, correct
the Kattis cpp language ID to `"C++"`, and absolutize the source file
path in `submit.lua` via `fnamemodify(..., ':p')` before spawning.

scrapers/language_ids.py

@@ -16,7 +16,7 @@ LANGUAGE_IDS = {
         "python": "python",
     },
     "kattis": {
-        "cpp": "C++17",
+        "cpp": "C++",
         "python": "Python 3",
     },
     "codechef": {