Add helper for converting a Stellar stroop amount to a human-readable XLM string with correct decimal places

[Chucks1093]

Summary

XLM amounts from the Stellar network arrive as stroop integers (1 XLM = 10,000,000 stroops). Several endpoints convert stroops to XLM inline with inconsistent decimal handling. A shared stroopsToXlm(stroops: bigint): string helper should centralise the conversion.

Scope

• Add stroopsToXlm(stroops: bigint): string returning a string with exactly 7 decimal places
• Add unit tests: 10000000 stroops → '1.0000000', 1 stroop → '0.0000001', 0 stroops → '0.0000000'
• Test a large value (e.g. 1000000000000 stroops) does not lose precision
• Replace all inline stroop-to-XLM conversions in existing endpoints with the helper
• Helper must not use floating-point arithmetic (use bigint throughout)

Acceptance Criteria

• 10,000,000 stroops returns '1.0000000'
• 1 stroop returns '0.0000001'
• 0 stroops returns '0.0000000'
• Large values handled without precision loss
• No floating-point arithmetic in the implementation

ETA: 12 hours

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