Habit tracker with streaks

Build a habit tracker for personal use.

Core features:
1. Add habits with cadence (daily / weekly / X times per week)
2. Check off from a today view, see current streak
3. Calendar heatmap per habit
4. Weekly recap
5. Push/email nudge when about to break streak

Data: habits, check_ins.
Must-haves: works well on mobile, offline-friendly, timezone-safe.