[SC-047] NairaToken: Approve function SEP-41 compliance

[ONEONUORA]

Description\nUpdate Naira Token approve function signature to support the standard expiration_ledger parameter.\n\n## Files to Edit/Create\n- contracts/contracts/naira_token/src/lib.rs\n\n## Acceptance Criteria\n- Signature must match approve(env, from, spender, amount, expiration_ledger).

• Verify current ledger sequence is less than expiration ledger.\n\n## Guidance / Hints\nConsult SEP-41 specifications in Stellar docs.

[drips-wave]

Congratulations, @innovativedesign-lab! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on July 30, 2026.

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