fix(codeforces): persist cookies after submit regardless of name (#312)

## Problem

The submit path in `codeforces.py` guarded `cookie_cache.write_text` on
the presence of a `JSESSIONID` cookie. Codeforces does not use
`JSESSIONID`, so the cookie file was never written after submit,
breaking the fast-path on subsequent submits.

## Solution

Replace the name-specific guard with a non-empty check (`if
browser_cookies:`), matching the unconditional save already used in the
login path.

Closes #301.

scrapers/codeforces.py

@@ -550,7 +550,7 @@ def _submit_headless(
 
             try:
                 browser_cookies = session.context.cookies()
-                if any(c["name"] == "JSESSIONID" for c in browser_cookies):
+                if browser_cookies:
                     cookie_cache.write_text(json.dumps(browser_cookies))
             except Exception:
                 pass