fix(scrapers): cookie fast paths, centralized storage, and reauth hardening (#363)

## Problem

Scraper cookie handling was fragmented across per-platform files with no
shared access, httpx scrapers lacked `checking_login` fast paths on
login, and several re-auth edge cases (CodeChef submit, CF cookie guard,
AtCoder cookie persistence) caused unnecessary full re-logins or silent
failures.

## Solution

Centralize all cookie storage into a single `cookies.json` via helpers
in `base.py`. Add `checking_login` fast paths to `kattis.py` (using the
`x-username` response header as a session probe), `usaco.py`, and
`cses.py` login flows. Fix `kattis.py` submit to emit `checking_login`
only after loading cookies. Remove AtCoder cookie persistence from login
entirely — always do a fresh session. Harden CodeChef and CF reauth
with consistent status logging and cookie guard checks.

scrapers/base.py

@@ -4,6 +4,38 @@ import os
 import re
 import sys
 from abc import ABC, abstractmethod
+from pathlib import Path
+from typing import Any
+
+_COOKIE_FILE = Path.home() / ".cache" / "cp-nvim" / "cookies.json"
+
+
+def load_platform_cookies(platform: str) -> Any | None:
+    try:
+        data = json.loads(_COOKIE_FILE.read_text())
+        return data.get(platform)
+    except Exception:
+        return None
+
+
+def save_platform_cookies(platform: str, data: Any) -> None:
+    _COOKIE_FILE.parent.mkdir(parents=True, exist_ok=True)
+    try:
+        existing = json.loads(_COOKIE_FILE.read_text())
+    except Exception:
+        existing = {}
+    existing[platform] = data
+    _COOKIE_FILE.write_text(json.dumps(existing))
+
+
+def clear_platform_cookies(platform: str) -> None:
+    try:
+        existing = json.loads(_COOKIE_FILE.read_text())
+        existing.pop(platform, None)
+        _COOKIE_FILE.write_text(json.dumps(existing))
+    except Exception:
+        pass
+
 
 from .language_ids import get_language_id
 from .models import (