Add helper for formatting XLM transaction amounts with sign prefix for activity feed

[Chucks1093]

Summary

The wallet activity feed displays both buy and sell amounts. Buys should show a negative sign (XLM spent) and sells a positive sign (XLM received). A shared helper should handle sign prefix, decimal formatting, and the XLM suffix consistently across all activity displays.

Scope

• Add formatActivityAmount(amount: bigint, type: 'buy' | 'sell'): string utility
• Buy: prefix with -, sell: prefix with +
• Format value in XLM with consistent decimal places using the existing formatKeyPrice helper
• Add unit tests for buy amount, sell amount, and zero amount

Acceptance Criteria

• Buy amount prefixed with -
• Sell amount prefixed with +
• Zero amount returns +0.00 XLM
• Unit tests cover all three cases

ETA: 12 hours

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Coordinate on Telegram

[lawalajose]

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

Would love to work on this

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[Olasunkanmi975]

@Olasunkanmi975 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

I have a solid understanding of the underlying architecture required for this task and am ready to start testing a solution immediately.

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