- Add _site() and route all URLs / Origin / Referer through _base_url()
- Problem list: use questionList on US (problemsetQuestionList on CN)
- Cookie validation: globalData.userStatus.isSignedIn on US, todayRecord on CN
- Skip interpret and block Submit on US (Cloudflare protects leetcode.com's
  state-changing endpoints); document US as read-only in the README