feat(codeforces): implement submit; cache CSES token (#300)

## Problem

Codeforces submit was a stub. CSES submit re-ran the full login flow on
every invocation (~1.5s overhead).

## Solution

**Codeforces**: headless browser submit via StealthySession (same
pattern as AtCoder). Solves Cloudflare Turnstile on login, uploads
source via file input, caches cookies at
`~/.cache/cp-nvim/codeforces-cookies.json` so repeat submits skip login.

**CSES**: persist the API token in credentials via a `credentials`
ndjson event. Subsequent submits validate the cached token with a single
GET before falling back to full login.

Also includes a vimdoc table of contents.

scrapers/codeforces.py

@@ -2,7 +2,9 @@
 
 import asyncio
 import json
+import os
 import re
+import tempfile
 from typing import Any
 
 import requests
@@ -18,10 +20,16 @@ from .models import (
     SubmitResult,
     TestCase,
 )
+from .timeouts import (
+    BROWSER_ELEMENT_WAIT,
+    BROWSER_NAV_TIMEOUT,
+    BROWSER_SESSION_TIMEOUT,
+    BROWSER_SETTLE_DELAY,
+    HTTP_TIMEOUT,
+)
 
 BASE_URL = "https://codeforces.com"
 API_CONTEST_LIST_URL = f"{BASE_URL}/api/contest.list"
-TIMEOUT_SECONDS = 30
 HEADERS = {
     "User-Agent": "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/124.0.0.0 Safari/537.36"
 }
@@ -136,7 +144,7 @@ def _is_interactive(block: Tag) -> bool:
 
 def _fetch_problems_html(contest_id: str) -> str:
     url = f"{BASE_URL}/contest/{contest_id}/problems"
-    response = curl_requests.get(url, impersonate="chrome", timeout=TIMEOUT_SECONDS)
+    response = curl_requests.get(url, impersonate="chrome", timeout=HTTP_TIMEOUT)
     response.raise_for_status()
     return response.text
 
@@ -223,7 +231,7 @@ class CodeforcesScraper(BaseScraper):
 
     async def scrape_contest_list(self) -> ContestListResult:
         try:
-            r = requests.get(API_CONTEST_LIST_URL, timeout=TIMEOUT_SECONDS)
+            r = requests.get(API_CONTEST_LIST_URL, timeout=HTTP_TIMEOUT)
             r.raise_for_status()
             data = r.json()
             if data.get("status") != "OK":
@@ -289,13 +297,187 @@ class CodeforcesScraper(BaseScraper):
         language_id: str,
         credentials: dict[str, str],
     ) -> SubmitResult:
+        return await asyncio.to_thread(
+            _submit_headless,
+            contest_id,
+            problem_id,
+            source_code,
+            language_id,
+            credentials,
+        )
+
+
+def _wait_for_gate_reload(page, wait_selector: str) -> None:
+    from .atcoder import _solve_turnstile
+
+    if "Verification" not in page.title():
+        return
+    _solve_turnstile(page)
+    page.wait_for_function(
+        f"() => !!document.querySelector('{wait_selector}')",
+        timeout=BROWSER_ELEMENT_WAIT,
+    )
+
+
+def _submit_headless(
+    contest_id: str,
+    problem_id: str,
+    source_code: str,
+    language_id: str,
+    credentials: dict[str, str],
+) -> SubmitResult:
+    from pathlib import Path
+
+    try:
+        from scrapling.fetchers import StealthySession  # type: ignore[import-untyped,unresolved-import]
+    except ImportError:
         return SubmitResult(
             success=False,
-            error="Codeforces submit not yet implemented",
-            submission_id="",
-            verdict="",
+            error="scrapling is required for Codeforces submit",
         )
 
+    from .atcoder import _ensure_browser, _solve_turnstile
+
+    _ensure_browser()
+
+    cookie_cache = Path.home() / ".cache" / "cp-nvim" / "codeforces-cookies.json"
+    cookie_cache.parent.mkdir(parents=True, exist_ok=True)
+    saved_cookies: list[dict[str, Any]] = []
+    if cookie_cache.exists():
+        try:
+            saved_cookies = json.loads(cookie_cache.read_text())
+        except Exception:
+            pass
+
+    logged_in = False
+    login_error: str | None = None
+    submit_error: str | None = None
+
+    def check_login(page):
+        nonlocal logged_in
+        logged_in = page.evaluate(
+            "() => Array.from(document.querySelectorAll('a'))"
+            ".some(a => a.textContent.includes('Logout'))"
+        )
+
+    def login_action(page):
+        nonlocal login_error
+        try:
+            _wait_for_gate_reload(page, "#enterForm")
+        except Exception:
+            pass
+        try:
+            page.fill(
+                'input[name="handleOrEmail"]',
+                credentials.get("username", ""),
+            )
+            page.fill(
+                'input[name="password"]',
+                credentials.get("password", ""),
+            )
+            page.locator('#enterForm input[type="submit"]').click()
+            page.wait_for_url(
+                lambda url: "/enter" not in url, timeout=BROWSER_NAV_TIMEOUT
+            )
+        except Exception as e:
+            login_error = str(e)
+
+    def submit_action(page):
+        nonlocal submit_error
+        try:
+            _solve_turnstile(page)
+        except Exception:
+            pass
+        tmp_path: str | None = None
+        try:
+            page.select_option(
+                'select[name="submittedProblemIndex"]',
+                problem_id.upper(),
+            )
+            page.select_option('select[name="programTypeId"]', language_id)
+            with tempfile.NamedTemporaryFile(
+                mode="w", suffix=".cpp", delete=False, prefix="cf_"
+            ) as tf:
+                tf.write(source_code)
+                tmp_path = tf.name
+            try:
+                page.set_input_files('input[name="sourceFile"]', tmp_path)
+                page.wait_for_timeout(BROWSER_SETTLE_DELAY)
+            except Exception:
+                page.fill('textarea[name="source"]', source_code)
+            page.locator("form.submit-form input.submit").click(no_wait_after=True)
+            try:
+                page.wait_for_url(
+                    lambda url: "/my" in url or "/status" in url,
+                    timeout=BROWSER_NAV_TIMEOUT * 2,
+                )
+            except Exception:
+                err_el = page.query_selector("span.error")
+                if err_el:
+                    submit_error = err_el.inner_text().strip()
+                else:
+                    submit_error = "Submit failed: page did not navigate"
+        except Exception as e:
+            submit_error = str(e)
+        finally:
+            if tmp_path:
+                try:
+                    os.unlink(tmp_path)
+                except OSError:
+                    pass
+
+    try:
+        with StealthySession(
+            headless=True,
+            timeout=BROWSER_SESSION_TIMEOUT,
+            google_search=False,
+            cookies=saved_cookies,
+        ) as session:
+            print(json.dumps({"status": "checking_login"}), flush=True)
+            session.fetch(
+                f"{BASE_URL}/",
+                page_action=check_login,
+                network_idle=True,
+            )
+
+            if not logged_in:
+                print(json.dumps({"status": "logging_in"}), flush=True)
+                session.fetch(
+                    f"{BASE_URL}/enter",
+                    page_action=login_action,
+                    solve_cloudflare=True,
+                )
+                if login_error:
+                    return SubmitResult(
+                        success=False, error=f"Login failed: {login_error}"
+                    )
+
+            print(json.dumps({"status": "submitting"}), flush=True)
+            session.fetch(
+                f"{BASE_URL}/contest/{contest_id}/submit",
+                page_action=submit_action,
+                solve_cloudflare=True,
+            )
+
+            try:
+                browser_cookies = session.context.cookies()
+                if any(c["name"] == "JSESSIONID" for c in browser_cookies):
+                    cookie_cache.write_text(json.dumps(browser_cookies))
+            except Exception:
+                pass
+
+            if submit_error:
+                return SubmitResult(success=False, error=submit_error)
+
+            return SubmitResult(
+                success=True,
+                error="",
+                submission_id="",
+                verdict="submitted",
+            )
+    except Exception as e:
+        return SubmitResult(success=False, error=str(e))
+
 
 if __name__ == "__main__":
     CodeforcesScraper().run_cli()