Build penalty tier calculator utility

[Kingsman-99]

Labels: complexity: high

Add a pure utility calculatePenalty(invoice, paymentTimestamp) that computes the penalty amount owed for a late payment based on the invoice's penalty_tiers configuration. This should run client-side without an RPC call.

Acceptance criteria:

• Takes an Invoice object and a Unix timestamp
• Returns { penaltyBps: number, penaltyAmount: bigint, tier: number | null }
• Returns zero penalty when payment is before penalty_deadline
• Matches contract on-chain penalty logic exactly
• Unit tested with at least 5 cases including edge cases

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[aliyuHabibu]

@aliyuHabibu has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, I'm a software engineer. I have gone through the issue and I can resolve it, assign it to me. Thank you

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[kokobutter-web]

@kokobutter-web has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I'm a full-stack developer and I'd like to take this issue.
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ETA: 1–3 days (depending on scope)

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