fix(scrapers): fix Kattis and USACO login and submit (#330)

## Problem

Kattis and USACO login and submit were broken in multiple ways
discovered
during manual end-to-end testing. Neither platform could successfully
authenticate or submit through the plugin.

## Solution

**Kattis:** switch login from `POST /login/email` (requires CSRF fetch)
to
`POST /login` with `script=true` (200 = success, 403 = bad credentials);
remove `_check_kattis_login` entirely since Kattis blocks all GET
requests
from httpx; add submit retry on `"Request validation failed"` to handle
expired sessions; fix language ID `"C++17"` → `"C++"`.

**USACO:** fix login field `user` → `uname`; fix success check to
`code==1`;
fix submit endpoint to `submit-solution.php`, file field to
`sourcefile`,
hidden field extraction off-by-one (`group(2)` → `group(1)`); fix
`_pick_lang_option` loop order (keywords outer, options inner) so
specific
keywords like `"c++17"` match before broad ones like `"c++"`.

**`submit.lua`:** absolutize source file path via `fnamemodify(...,
':p')`
before passing to the scraper — Python is spawned with `cwd=plugin_path`
so relative paths silently fail with `FileNotFoundError`.

**Both platforms:** remove cookie fast path from `login` subcommand so
credentials are always validated, preventing stale cookies from masking
wrong credentials.

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()