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.

lua/cp/submit.lua

@@ -54,6 +54,7 @@ function M.submit(opts)
     logger.log('Source file not found', { level = vim.log.levels.ERROR })
     return
   end
+  source_file = vim.fn.fnamemodify(source_file, ':p')
 
   prompt_credentials(platform, function(creds)
     vim.cmd.update()