feat: add research-based strategy lab

[LessUp]

Summary

• add a Strategy Lab entry point with research-inspired presets (spacing + recall, chunking, dual-coding, retrieval pressure) and in-round badges/notes
• apply chosen strategies to decks, previews, hints, countdown scaling, and i18n surfaces to guide practice
• document the Strategy Lab option alongside existing modes in the README

Testing

• npm test -- --runInBand

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Codex Task