Add SDK payment preview helper

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Add a payment preview helper for mobile and web confirmation screens.

Current Behaviour

Apps need a safe preview of payment details before signing or submission.

Expected Behaviour

The SDK should return a typed preview including source, destination, amount, asset, memo, network, and estimated fee where available.

Suggested Implementation

Implement or design a preview helper that does not sign or submit transactions.

Files or Areas Likely Affected

src/payments/
src/transactions/
src/types/
tests/
docs/

Acceptance Criteria

• Payment preview helper is added.
• Preview includes key payment details.
• Helper does not sign or submit.
• Tests cover native payment preview.
• Documentation includes UI usage example.

Additional Notes

This aligns with mobile transaction review work.

[adnaan-id]

Let me try this, maintainer. You'll receive a pr in less than a day

[grantfox-oss]

🦊 GrantFox — @adnaan-id has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #242)
2. Your PR will be reviewed by the Axionvera maintainers

Good luck! Track your progress on GrantFox.