feat: enhance drill mode with scoring, question browser, and timer fixes

- Add scoring system based on speed, difficulty, and streak
- Add question browser with search filter for selecting specific questions
- Add per-question stopwatch timer in drill mode
- Fix quiz timer not resetting between questions
- Hide preset buttons by default in QuestionCountSlider
- Add cursor-pointer to all interactive elements across drill and quiz modes
- Display points earned in feedback and total score in results

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: davidagustin