Add helper for formatting key price display in XLM with correct decimal places

[Chucks1093]

Summary

Key prices from the contract are returned in stroops. The client needs a shared formatting helper that converts stroops to XLM and displays the result with a consistent number of decimal places across all price displays.

Scope

• Add a formatKeyPrice(stroops: bigint): string utility function
• Always display 2 decimal places for prices above 1 XLM, 4 decimal places for prices below 1 XLM
• Add unit tests covering: zero, sub-1 XLM, exactly 1 XLM, large values

Acceptance Criteria

• Helper converts stroops to XLM correctly
• Decimal place rules are applied correctly
• Unit tests pass for all boundary cases

ETA: 12 hours

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Coordinate on Telegram

[Joseph-1-Duro]

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Hi team, I would love to work on your project.

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[Oluwasuyi-Oluwatimilehin-Daniel]

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

Hello maintainers I'm a full stack developer experience in building scalable systems I can handle this perfectly if assigned

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[jotel-dev]

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pls assign me

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[grantfox-oss]

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@jotel-dev's PR #434 was approved and merged by @Chucks1093.

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