refactor: consolidate stroopsToXlm into stellar-format.ts and add precision test

[Tekh134]

Consolidates the stroopsToXlm helper into the canonical bigint-based implementation in src/lib/stellar-format.ts.

• Removes the duplicate Decimal.js-based stroopsToXlm from src/utils/unit-conversion.utils.ts
• Re-exports stroopsToXlm from stellar-format.ts for backward compatibility
• Adds explicit test for 1,000,000,000,000 stroops (100,000 XLM) in stellar-format.test.ts
• Updates unit-conversion tests to import from the canonical implementation
• Ensures all stroop-to-XLM conversions use bigint arithmetic throughout

Closes #185

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