test: add comprehensive unit tests for formatting utilities and updat…

[Unclebaffa]

📝 Comprehensive Summary of Implementation

1. Formatting Utilities Unit Test Suite (format.test.js)

Expanded unit test coverage from 3 basic functions to 26 unit tests covering all 7 functions exported by src/utils/format.js:

• formatAmount:
  • Default en-US formatting ($1,234.50).
  • Custom locales (fr-FR decimal/grouping separators, en-GB pound symbol placement).
  • Fallback to $0.00 for non-numeric amounts ('not-a-number').
  • Numeric string inputs ('100.5' -> $100.50).
  • Defaulting omitted locale arguments.
• formatDate:
  • Default en-US date formatting (Jul 15, 2026).
  • Custom locales (15 juil. 2026 for fr-FR).
  • Missing value placeholders ('-') for null, undefined, or empty string ''.
  • Date objects and numeric timestamps.
• formatRate:
  • 4-decimal place rate formatting (1 USD = 294.6200 NGN).
  • Placeholder handling ('-') for null or undefined rates.
• formatCurrencyInput:
  • Empty output ('') for null, undefined, empty string '', or single period '.'.
  • Stripping non-numeric characters and formatting to 2 decimal places ('1,234.5' -> '1234.50', '$100.5' -> '100.50').
  • Number and numeric string inputs.
  • Empty string output ('') for non-numeric input strings ('abc').
• formatPercent:
  • Ratio to percentage conversion with 2 decimal places default (0.005 -> '0.50%').
  • Custom decimal places (formatPercent(0.005, 1) -> '0.5%').
  • Fallbacks to '0.00%' for invalid/missing inputs.
• formatNumber:
  • Default en-US grouping (1,234.5).
  • Custom locale grouping and decimal separators (fr-FR).
  • Fallbacks to '0' for non-numeric inputs.
• shortenAddress:
  • Default Stellar public key truncation (GBQAZ7...CDEF).
  • Custom head and tail lengths (shortenAddress(address, 4, 3) -> GBQA...DEF).
  • Preservation of strings where length $\le$ head + tail.
  • Placeholder ('-') for empty, null, or undefined inputs.

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2. Documentation Updates (README.md)

• Documented the unit test suite for formatting utilities under the Testing section in README.md.

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3. Code Quality & Verification

• Vitest Suite: All 26 unit tests in test/unit/format.test.js passed (and 144/144 tests passed across the complete test suite).
• ESLint (npm run lint): 0 errors, 0 warnings.
• Prettier (npm run format:check): 100% code style compliant.

Closes #9