- Add calculatePreviewIntervals for Again/Hard/Good/Easy buttons
- Support NEW, LEARNING, RELEARNING, and REVIEW card types
- Use SM2_CONFIG constants for accurate interval calculation
- All intervals returned in minutes for consistent formatting
Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>
