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.

lua/cp/constants.lua

@@ -219,4 +219,6 @@ M.LANGUAGE_VERSIONS = {
 
 M.DEFAULT_VERSIONS = { cpp = 'c++20', python = 'python3' }
 
+M.COOKIE_FILE = vim.fn.expand('~/.cache/cp-nvim/cookies.json')
+
 return M