core: implement loan application credit simulation

[EmeditWeb]

Problem

Before applying for a loan learners have no way to
simulate how the loan will affect their finances.

What To Build

1. Credit simulator screen with sliders
2. Affordability advice (green/amber/red)
3. Comparison view (3 vs 6 vs 12 months)
4. Save simulation

Files To Touch

• app/(tabs)/simulate.tsx (new)
• hooks/useLoanSimulator.ts (new)
• app/(tabs)/_layout.tsx

Acceptance Criteria

• All calculations correct and real-time
• Income health indicator accurate
• Comparison table renders correctly
• Saved simulation pre-fills wizard
• npx expo export --platform web passes

Mandatory Checks Before PR

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

[mansur-codes]

I can effectively deliver the fix to this issue, please assign, thank you.

[grantfox-oss]

🦊 GrantFox — @mansur-codes 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 #18)
2. Your PR will be reviewed by the StepFi-app maintainers

Good luck! Track your progress on GrantFox.