core: implement installment calendar with reminders

[EmeditWeb]

Problem

Learners have no visual calendar of upcoming payment
dates. They miss payments because they forget.

What To Build

1. Calendar view with due date markers
2. Device calendar integration
3. Enhanced notification schedule (7/3/1 day before)
4. Payment streak tracking

Files To Touch

• app/(tabs)/calendar.tsx (new)
• hooks/useInstallmentCalendar.ts (new)
• services/notifications.service.ts
• app/(tabs)/_layout.tsx

Acceptance Criteria

• Calendar shows all due dates
• Tap shows payment amount and loan name
• Calendar export works with permission
• 4-stage notification schedule works
• npx expo export --platform web passes

Mandatory Checks Before PR

• npx expo export --platform web passes
• PR references this issue

[fadesany]

Hello maintainer, i would like to work on this issue

[EmeditWeb]

Hello maintainer, i would like to work on this issue

all files in the context folder should be put into consideration when working on the issue. Thanks

[grantfox-oss]

🦊 GrantFox — @fadesany has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #16)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@fadesany's PR #22 was approved and merged by @EmeditWeb.

🏆 @fadesany: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (118 total points). Track your full progress on GrantFox.

👏 Great work, @fadesany! Keep contributing to StepFi-app.