test(format): Add unit tests for XLM amount formatter (#39)

[abdulqoyumadegoke]

Closes #39

Overview

Resolves #39 by adding a standardized formatXLM amount formatting utility function (lib/format.ts) and unit test suite (lib/__tests__/formatXLM.test.ts) that guarantees correct thousand separators, two fixed decimal places, and accurate handling of zero, fractional, and negative XLM values without floating-point representation errors.

Key Changes

• lib/format.ts: Implemented formatXLM utility handling number | string | bigint inputs, appending trailing XLM suffixes with en-US thousand separators (1,000,000.00 XLM) and fallback safety.
• lib/portfolio.ts: Updated portfolio summary total calculation to use formatXLM.
• lib/__tests__/formatXLM.test.ts:
  • Added unit test cases verifying 10000 -> '10,000.00 XLM'.
  • Added unit test cases verifying 0 -> '0.00 XLM'.
  • Added unit test cases verifying 0.5 -> '0.50 XLM'.
  • Added unit test cases verifying 1000000 -> '1,000,000.00 XLM'.
  • Added unit test cases verifying negative values (-500) -> '-500.00 XLM'.
  • Verified floating-point rounding precision (0.1 + 0.2 -> '0.30 XLM', 1234.5678 -> '1,234.57 XLM').

Verification

• Verified unit test suite assertions.

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