add habit goal-period difficulty and habit formation research notes

Ground the 21-day habit myth in actual research (Lally et al. 2010) and let users pick a target period (21/66/254 days or unlimited) matching that study's easy/median/hard automaticity timelines when creating a habit, with progress shown on the habits list. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
2026-07-19 10:12:17 +09:00
parent 55918d56d1
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 개인용 습관 관리 PWA. 아이폰과 PC에서 같은 서버(MariaDB)에 접속해 습관을 관리합니다.

+참고: 습관이 실제로 어떻게 형성되는지에 대한 연구 정리는 [habit-formation-research.md](./habit-formation-research.md) 참고.
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 ## 설치

 ```bash