Build invoice multi-currency support

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Global teams need to pay invoices in their local Stellar assets (XLM, USDC, EURC) without manual conversion. This issue adds accepted_tokens: Vec<Address> to create_invoice() — pay() accepts any listed token, converting it to the invoice base token via DEX before crediting.

Technical Context

Involves types.rs (add accepted_tokens: Vec<Address>) and lib.rs. Add pay_with_token(env, payer, invoice_id, source_token, amount). Assert source_token is in accepted_tokens. If source_token != invoice.token, invoke DEX swap. Credit converted amount to invoice.funded.

Acceptance Criteria

• create_invoice() accepts accepted_tokens list
• pay_with_token() accepts any token in the list
• Non-listed token panics with "token not accepted"
• Accepted non-base token swapped to base token before crediting
• Test pays with alternate token and verifies correct USDC amount credited
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[Amas-01]

@Amas-01 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hello Maintainer
I'll Like to work on this Issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Amas-01 to this issue.

[james2177]

@james2177 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer, i would like to work on this issue

ℹ️ Repo Maintainers: To accept this application, review their application or assign @james2177 to this issue.

[drips-wave]

Congratulations, @james2177! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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