Add currency_iso validation

[heymide]

Summary

Added strict currency validation so only NGN and USD are accepted, and invalid values now return 400 with a stable error message.

Related Issue

Closes #167

The shared whitelist lives in [schema.ts] and is enforced by the runtime validator in [validation.ts]. Gift creation now uses that validator directly in [route.ts], and public gift creation rejects unsupported currencies with 400 in [route.ts]. I also changed the old unsupported default away from USDC to NGN, including the legacy schema mirror in [schema.ts].

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the [Zendvo Five Principles]
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

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[heymide]

Done, Close: #185

[Emrys02]

@heymide PR reviewed

[Emrys02]

@heymide please resolve conflicts

[heymide]

@heymide please resolve conflicts

Done