fix: preserve zero-decimal currency precision in amount conversion

[Zenoxy97]

The amount conversion helper replaces a valid zero decimal precision with the two-decimal fallback. For 1,000 JPY with an 8% fee, converting a decimal amount to minor units returns 92,000 instead of 920. The inverse conversion displays 9.2 instead of 920. The seller-net calculation also receives the incorrect minor-unit amount.

This changes decimalPlaces || 2 to decimalPlaces ?? 2, preserving zero precision while retaining the fallback for unknown currencies. Regression tests cover zero-decimal currencies in both directions, uppercase JPY, the unknown-currency fallback, and the seller-net calculation. Existing two- and three-decimal tests remain green.

Validation on the current master base:

• Targeted amount-conversion and seller-net suites: 36 passing.
• TypeScript: npm run typecheck passes.
• Full CircleCI pipeline: build, database migrations/seeds, backend tests, and frontend tests passed on commit 3866061 (run #4987).
• No real payments were exercised; this does not establish that production transactions were affected.

This contribution was prepared with OpenAI Codex assistance and independently reviewed by a second coding agent.

Would you consider a USD 25 bounty for accepting and merging this fix? No bounty was previously agreed; this is an optional request. Payment details can be shared privately after agreement.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-09T14:53:39.961839Z	3866061	PR opened

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[alexanmtz]

Thanks for the fix. To be upfront: we don't have budget for bounties right now, so there's no bounty funded for this PR and I can't confirm the $25 ask. Once we do have budget, we'll add funded issues on the platform, and/or set up a program to pay out for useful PRs like this one — so it's worth checking back.

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That said, we're still happy to review this PR on its own merits if you'd like us to — no payment obligation either way. If you'd only like it merged in exchange for payment, totally understandable, and you're welcome to close it instead; no hard feelings.

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