Add tab routing, open-time countdown w/ auto-start, invite links, class stats

- Hash routes per tab (#/overview, #/bank, #/papers, #/analytics, #/settings)
- Student lobby: live open-time countdown that auto-starts the paper at zero
- Copy paper invite link (?code=...) that auto-selects the paper for students
- Analytics sub-tabs + Class stats (score distribution, accuracy by difficulty,
  cohort trend, most-missed item analysis); grade-wise leaderboard
- Paper editor: lesson-level + expandable question-level selection
- Fix dark-theme sidebar text and paper-choice card titles

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
