Add the question bank with a system design round
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Commit

- 56 questions: 10 behavioural, 6 HR, 10 technical per track (frontend, backend, ML) and 10 system design, each with intent, key points, what a strong answer covers and written follow-ups
- System design is its own round, junior and up
- pickQuestions() for a round or a full loop
Entire-Checkpoint: 01M41N85BADWP82FJ4B68KPTTN
Checkpoints
Checkpoint 1