core: add overflow-safe arithmetic across all contracts

[EmeditWeb]

Problem

Multiple contracts perform arithmetic on i128 amounts
without overflow checks. A carefully crafted large
loan amount or share calculation could overflow.

What To Build

1. Create safe_math module in each contract
2. Replace all +, -, * operations on financial values

Files To Touch

All 5 contract lib.rs and errors.rs files

Acceptance Criteria

• All financial arithmetic uses checked ops
• Overflow returns typed error not panic
• Underflow returns typed error not panic
• Tests with extreme values pass safely

Mandatory Checks Before PR

• cargo build passes
• All tests pass
• PR references this issue

[Dopezapha]

Hey maintainer, I would like to work on this by adding a reusable safe_math module to each contract and replacing financial +, -, and * operations on i128 values with checked arithmetic that returns typed contract errors instead of panicking or overflowing.
I’ll update the relevant lib.rs and errors.rs files, add extreme-value tests for overflow and underflow paths, and verify the full contract suite with cargo build and all tests before opening a PR that references this issue.

[grantfox-oss]

🦊 GrantFox — @Dopezapha 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 #20)
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!

@Dopezapha's PR #29 was approved and merged by @EmeditWeb.

🏆 @Dopezapha: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (433 total points). Track your full progress on GrantFox.

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