core: implement transaction signing abstraction layer

[EmeditWeb]

Problem

Transaction signing logic is duplicated across loan
repayment, vouch submission, and deposit flows.

What To Build

1. TransactionSigner service
2. useTransaction hook with consistent states
3. Replace all inline signing with abstraction

Files To Touch

• src/transactions/transaction-signer.service.ts (new)
• hooks/useTransaction.ts (new)
• All screens with transaction signing

Acceptance Criteria

• All signing routes through TransactionSigner
• Consistent states across all flows
• User rejection handled gracefully
• No duplicate signing logic anywhere
• npx expo export --platform web passes

Mandatory Checks Before PR

• npx expo export --platform web passes
• No any types
• Unit tests for TransactionSigner
• PR references this issue

[fadesany]

hello maintainer, i would like to work on this issue

[Adeyemi-cmd]

Can i work on this please

[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 #21)
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 #25 was approved and merged by @EmeditWeb.

🏆 @fadesany: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (153 total points). Track your full progress on GrantFox.

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